Electrochemical sensors and methods for using electrochemical sensors to detect plant pathogen infection

ABSTRACT

Provided herein are plant/plant pathogen volatile compound electrochemical sensors, plant/plant pathogen volatile detection systems, and methods for detecting stress-induced plant volatile compounds and/or a plant-pathogen emitted volatile compounds.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.15/566,438 filed on Oct. 13, 2017, which application is the 35 U.S.C. §371 national stage application of PCT Application No. PCT/US2016/027735,filed Apr. 15, 2016, where the PCT claims the benefit of and priority toU.S. Provisional Patent Application No. 62/147,763, filed on Apr. 15,2015, entitled “ELECTROCHEMICAL SENSORS AND METHODS FOR USINGELECTROCHEMICAL SENSORS TO DETECT PLANT PATHOGEN INFECTION,” each ofwhich are herein incorporated by reference in their entireties.

FEDERAL SPONSORSHIP

This invention was made with Government support under Grant No.CBET-1159540 awarded by the National Science Foundation, Grant No.045097-01 awarded by the American Chemical Society, and Grant No.2015-67021-23188 awarded by the National Institute of Food andAgriculture. The Government has certain rights in this invention.

BACKGROUND

Agricultural losses due to plant pathogen infestations are estimated at$40 billion annually in the U.S. alone. Advanced disease detection andprevention in crops is a challenge for sustainable agriculture. Amongthe various types of pathogens that attack crops, fungi are the mostcommon and most devastating for crops from Michigan to Florida in theeast coast to California, Arizona and Mexico in the west. Pathogenicfungal infections cause numerous diseases such as white mold, grey mold,crown rot, leaf blight, fruit rot etc. in a variety of grain, fruit, andvegetable crops. For example, Phytophthora capsici alone is known toinfect as many as 68 crops from 27 different families across the U.S.

The infections occur at multiple sites in crops (e.g., root, leaf, stem,fruit) during growth or after harvest, such as when produce is beingstored or transported. If not controlled early, these infections spreadquickly by wind, water, or physical contact between plants, causingdevastating economic loss. The narrow profit margin greatly limitsproducers' options for effective controls for these diseases. Economicmanagement of fruits may be challenging, as fruits are exposed to fungalinoculum for an extended period of time. Due to the lack ofearly-detection technology, fungicide application (even multipleapplications) may come late and prove ineffective, resulting in almost90% grower losses in some cases. Frequently these crops are sold throughmass distribution well before the infections are known. Therefore, anearly detection of pathogen infections could help growers contain theinfection, spray only when needed, and minimize economical losses.

Currently used methods for disease detection in agricultural cropsinclude direct methods, such as pathogen isolation and identificationbased on morphological characteristics, polymerase chain reaction (PCR),fluorescence in-situ hybridization (FISH), immunofluorescence (IF),enzyme-linked immuno-sorbent assay (ELISA), and gas chromatography massspectrometry (GC-MS). There are also indirect methods, such as hyperspectral imaging, fluorescence, and other spectroscopy based techniques.These methods are time consuming, destructive, demand skilled analysts,require a laboratory set-up, and, unfortunately, do not offer eitherreal-time monitoring or on-field deployment possibilities. Due to theirdestructive nature, direct methods can only be employed after the onsetof disease symptoms to verify the infection, and thus do not allow forearly monitoring and prevention. The indirect methods are expensive, donot possess high selectivity towards the infection/disease, and areprimarily effective for post-harvest evaluation.

SUMMARY

The present disclosure provides electrochemical sensors for detectingtarget stress-induced plant volatile compounds and/or targetpathogen-emitted volatile compounds, plant volatile detection systems,and methods for monitoring the condition of a plant or crop of plants.

Embodiments of electrochemical sensors include a volatile detectionelectrode. Embodiments of the volatile detection electrode include anelectrode substrate and a bio-nanocomposite detection element on asurface of the electrode substrate and in electrochemical communicationwith the electrode substrate. In embodiments, the bio-nanocompositedetection element includes a nanomaterial transducer material and one ormore enzymes capable of specific reaction with a target volatilecompound or its hydrolysis product, wherein the target volatile compoundis a stress-induced plant volatile compound or a target pathogen-emittedvolatile compound. In embodiments, the enzyme is immobilized on thenanomaterial transducer material, reaction between the enzyme and thetarget volatile compound generates an electrical signal, and detectionof the electrical signal indicates the presence of the target volatilecompound. In embodiments, the volatile detection electrode includes abi-enzyme or a tri-enzyme system, where at least one enzyme reacts withthe target volatile compound producing a cascade of reactions involvingthe other enzymes of the system, where at least one of the reactions(e.g., the final reaction in the cascade) produces an electrical signalcapable of being detected by the electrode.

Embodiments of a plant volatile detection system include: (a) a volatilecollection reservoir adapted to collect volatile compounds emitted froma plant; (b) an electrochemical sensor according to the presentdisclosure; and (c) a signal processing mechanism in operativecommunication with one or more elements of the electrochemical sensor,the signal processing mechanism having data transfer and evaluationsoftware protocols configured to transform raw data from theelectrochemical sensor into diagnostic information regarding thepresence or absence or levels of the plant volatile compound. Inembodiments, an electrochemical sensor of the plant volatile detectionsystem of the present disclosure includes an electrochemical cellincluding a volatile detection electrode, a counter electrode and areference electrode, both the counter electrode and reference electrodein electrochemical communication with the volatile detection electrode,and a potentiostat to supply an electric current to the electrochemicalcell and monitor changes in the electric current produced at thevolatile detection electrode. The volatile detection electrode of theelectrochemical cell is in fluid communication with the volatilecollection reservoir of the system such that volatile compoundscollected in the reservoir can be transferred to a detection surface ofthe volatile detection electrode. In embodiments, the volatile detectionelectrode has an electrode substrate and a bio-nanocomposite detectionelement on a detection surface of the electrode substrate and inelectrochemical communication with the electrode substrate, thebio-nanocomposite detection element having a nanomaterial transducermaterial and one or more enzymes capable of specific reaction with astress-induced plant volatile compound or its hydrolysis product, aplant-pathogen emitted volatile compound or its hydrolysis product, orboth, wherein the enzyme is immobilized to the nanomaterial transducermaterial.

The present disclosure also provides methods for monitoring a conditionof a plant or crop of plants using the electrochemical sensors and/orplant volatile detection systems of the present disclosure. Inembodiments, such methods can include periodically sampling volatileemissions from the plant or one or more crop plants using a plantvolatile detection system of the present disclosure and determining thepresence of a plant disease associated with the one or more volatilecompounds based on the information provided by a signal processingmechanism regarding the presence or absence or levels of the plantvolatile compound.

BRIEF DESCRIPTION OF THE DRAWINGS

Further aspects of the present disclosure will be more readilyappreciated upon review of the detailed description of its variousembodiments, described below, when taken in conjunction with theaccompanying drawings.

FIG. 1 illustrates a web showing the production of various volatilecompounds by or as a result of infection of various fruits andvegetables by pathogens such as Fusarium spp., Phytophthora sp., andScelrotium spp.

FIG. 2A illustrates cyclic voltammetry (CV) responses of ethyl phenoland ethyl guaiacol on an embodiment of a CNT/horseradish peroxidase(HRP) modified electrode. FIG. 2B illustrates CV responses of hydrolyzedmethyl salicylate (salicylate and methanol) on various modifiedelectrodes having no enzyme, 1 enzyme, and 2 enzymes (CNT only, CNT-AO,CNT-HRP, and CNT-AO/HRP).

FIGS. 3A and 3B are schematic illustrations of electron transfer betweenan enzyme and the electrode surface, showing mediator facilitatedelectron transfer (FIG. 3A) vs. direct electron transfer (FIG. 3B).

FIGS. 4A-4C are schematic illustrations of embodiments of a bi-enzymemodified electrode for detection of methyl salicylate (FIG. 4A), amono-enzyme modified electrode for detection of ethyl guaiacol or ethylphenol (FIG. 4B), and a mono-enzyme modified electrode for detection ofoctanone or octanol (FIG. 4C).

FIG. 5A illustrates an embodiment of an electrode modified withenzyme-nanoparticle composites, and FIG. 5B illustrates an embodiment ofa process of enzyme immobilization to metal oxide nanoparticles (MOxNPs).

FIG. 6 illustrates embodiments of an electrode modified with a threedimensional matrix of carbon nanotubes functionalized with enzymesattached to the CNTs with a molecular tether.

FIGS. 7A and 7B illustrate an embodiment of a method of functionalizinga CNT surface with enzymes using cross-linked PVA polymers totether/entrap the enzymes to form a CNT/enzyme 3D matrix on an electrodesurface.

FIG. 8 are AFM digital images illustrating the difference in morphologybetween unmodified (top) and enzyme modified (bottom) CNT surfaces.

FIG. 9 is a schematic illustration of methyl salicylate detection by anembodiment of a bi-enzyme modified electrode with alcohol oxidase (AO)and horseradish peroxidase (HRP). The process carries out as hydrolysisof methyl salicylate to form salicylic acid and methanol (1), oxidationof methanol and production of hydrogen peroxide (2) and direct electrontransfer from electrode to hydrogen peroxide by horseradish peroxidase(3).

FIG. 10 is a schematic illustration of a method of preparing anembodiment of an enzyme modified sensor of the present disclosureshowing coating an electrode substrate with a multiwall carbon nanotubetransducer material, modifying the MWCNT coated substrate with PBSE, andfinally functionalizing the prepared electrode with enzymes AO and HRP.

FIGS. 11A and 11B illustrate cyclic voltammetry (CV) responses of 3 mMmethanol on bi-enzyme modified, AO modified and HRP modified electrodes(FIG. 11A) and 1.88 mM hydrolyzed methyl salicylate on bi-enzymemodified, AO modified, HRP modified and no enzyme modified electrodes(FIG. 11B).

FIG. 12 illustrates CV responses of 5 mM formaldehyde and 5 mM salicylicacid on a monoenzyme (HRP or AO) modified electrode.

FIG. 13 illustrates CV responses of 1.88 hydrolyzed methyl salicylate onGC electrodes modified by a different ratio of AO and HRP enzymes.

FIGS. 14A and 14B illustrate CV responses of hydrolyzed methylsalicylate from 0, 1 μM to 3 mM on a bi-enzyme modified GC electrode(FIG. 14A) (insert shows current density versus concentration) andameprometric l-t curve of hydrolyzed methyl salicylate from 0, 0.1 to 1mM on a bi-enzyme modified RDE (FIG. 14B) (insert shows current densityversus concentration).

FIGS. 15A and 15B illustrate interference of cis-3-hexenol, hexylacetate, and cis-3-hexenyl acetate with phosphate buffer as control andinterference percentage dependent on concentration of green leafvolatiles (insert) (FIG. 15A) and display of details from 500 second to800 second (FIG. 15B).

FIG. 16 illustrates the stability of a bi-enzyme modified electrode with50 μM hydrolyzed methyl salicylate from Day 0 to Day 10 and currentdensity retained against time (insert).

FIG. 17 illustrates the repeatability of a bi-enzyme modified electrodewith 50 μM hydrolyzed methyl salicylate.

FIG. 18 illustrates the ameprometric I-t curve of hydrolyzed wintergreenoil from 0, 0.1 to 1 mM on a bi-enzyme modified RDE.

FIG. 19A illustrates cyclic voltammetry responses of SnO₂-SP (a and a′)and TiO₂-SP (b and b′) with (a and b) and without (a′ and b′) thepresence of 0.17 mM p-ethylguaiacol (FIG. 19A). FIGS. 19B and 19Cillustrate the concentration effect of ¬p-ethylguaiacol at SnO₂-SP (FIG.19B) and TiO₂-SP (FIG. 19C) electrodes.

FIG. 20A illustrates differential pulse voltammetry (DPV) responses ofSnO₂-SP (a and a′) and TiO₂-SP (b and b′) with (a and b) and without (a′and b′) the presence of 0.17 mM p-ethylguaiacol. FIGS. 20B and 20Cillustrate concentration effect of p-ethylguaiacol on SnO₂-SP (FIG. 20B)and TiO₂-SP (FIG. 20C) electrodes.

FIGS. 21A and 21B illustrate an interference study of 20.8 μMp-ethylguaicol with 6 different compounds p-ethylphenol,cis-3-hexen-1-ol, hexyl acetate and cis-3-hexen-1-yl acetate, 3-octanoneand 1-octen-3-ol on SnO₂-SP (FIG. 21A) and TiO₂-SP (FIG. 21B) by DPV.

FIG. 22 is a schematic illustration of methyl salicylate detection onbi-enzyme (salicylate hydroxylase and tyrosinase) based carbon nanotubeand PBSE modified biosensor. Methyl salicylate was hydrolyzed manuallyto generate salicylate and methanol (1). Salicylate, the main analytewas catalyzed by salicylate hydroxylase to generate catechol in presenceof NADH and oxygen (2). Catechol is oxidized by tyrosinase to form1,2-benzoquinone (3). The detection of methyl salicylate is finallyrealized by measuring the reduction of 1,2-benzoquinone to catechol onthe electrode surface (4).

FIG. 23A illustrates cyclic voltammetry after sequential addition of 100μL of 0.1 mM FAD, 50 μL of 10 mM NADH, and 25 μM salicylate, onsalicylate hydroxylase immobilized mono-enzyme CNT electrode includingSH. FIG. 23B illustrates cyclic voltammetry after the same additionscarried out on bi-enzyme CNT electrode including SH and TYR. Thereduction wave appearing below 0.15 V can be attributed to benzoquinonereduction described as step 4 in FIG. 22.

FIGS. 24A-24B illustrate cyclic voltammetry responses of unimmobilizedand TYR-immobilized mono-enzyme CNT electrodes in the presence andabsence of catechol (FIG. 24A) and salicylate (FIG. 24B) in theelectrolyte.

FIG. 25 illustrates cyclic voltammetry responses of the bi-enzymebiosensor containing immobilized salicylate hydroxylase and tyrosinase.The 2 mL electrolyte includes 25 μM salicylate with FAD (4.7 μM) andNADH (0.23 mM). The ratio of SH:TYR loadings by volume on the electroderespectively are 1:9, 3:7, 5:5, 7:3, and 9:1. Insert figure shows thecurrent density of the sensor measured at 0.025 V for different enzymeloading ratios showing the maximum sensitivity was obtained when theenzyme volume ratio was 1:1.

FIGS. 26A and 26B illustrate cyclic voltammetry (FIG. 26A) and constantpotential amperometry (FIG. 26B) responses of salicylate with presenceof FAD and NADH and sensitivity, linear range and R² value (Insert).

FIGS. 27A and 27B illustrate the reusability (27A) and stability (27B)of an embodiment of a bi-enzyme biosensor for salicylate from 2.3 μM to46.3 μM (current plots are in reverse order: Test 1 is lowest line plotand Test 10 is the top line plot for FIG. 27A; Day 1 is lowest plot andDay 1 is top-most plot for FIG. 27B). Current density retention overmeasurements in reusability (FIG. 27A) and time in stability (FIG. 27B)are displayed in insets with two low concentrations (2.3 μM and 4.6 μM)and high concentrations (18.5 μM and 27.8 μM) (line plots in bothinserts: top (4.6), middle-high: 9.3; middle-low: 18.5; bottom: 27.8)

FIG. 28 illustrates constant potential amperometry responses ofinterference compounds: methanol (MeOH), farnesene (FAR), humulene(HUM), dichlorobenzene (DCB), 1,2,3-trichlorobenze (TCB) and that of thecontrol (no interfering compound). The insert graph shows the linearincrease in farnesene currents with its concentration with a sensitivityof 0.042 μA·cm⁻²·μM⁻¹.

FIG. 29 illustrates constant potential amperometry responses ofuninfested synthetic analyte and infested synthetic analyte in simulatedsample study. The insert graph shows the sensitivity of the infectedsynthetic analyte and pure salicylate.

FIG. 30 is a schematic illustration of methyl salicylate detection on anembodiment of a tri-enzyme (tannase, salicylate hydroxylase andtyrosinase) based carbon nanotube and PBSE modified biosensor, where theimmobilized enzyme tannase hydrolyses methyl salicylate to salicylateand methanol in step (1).

FIG. 31 is a schematic illustration of methyl salicylate detection on anembodiment of a tri-enzyme (esterase, salicylate hydroxylase andtyrosinase) based carbon nanotube and PBSE modified biosensor where theimmobilized enzyme esterase hydrolyses methyl salicylate to salicylateand methanol in step (1).

FIGS. 32A-32D illustrate a series of cyclic voltammetry responses ofmethyl salicylate (FIGS. 32 A,B) and salicylate (FIGS. 32C,D) onsalicylate hydroxylase (SH) and tyrosinase (TRY) immobilized bienzymaticbiosensor (FIGS. 32A,C) and esterase (ES), SH and TYR immobilizedtrienzymatic biosensors (FIGS. 32 B,D).

FIG. 33 illustrates cyclic voltammetry responses of 92 uM methylsalicylate on trienzymatic biosensor with different volume ratios ofesterase (5 mg/mL), salicylate hydroxylase and tyrosinase (5 mg/mL) andthe net current density against different esterase volume percentage(Insert).

FIGS. 34A and 34B illustrate cyclic voltammetry (FIG. 34A) and constantpotential amperometry (FIG. 34B) responses of a tri-enzymatic biosensorto different concentrations of methyl salicylate. Inserted graphs arecurrent density versus concentration. Sensitivity is determined to be0.78 μA·cm⁻²·μM and 1.34 μA·cm⁻²·μM by CV and CPA, respectively.

FIG. 35 is a schematic illustration of p-ethylphenol detection on anembodiment of a tyrosinase-immobilized biosensor. p-ethylphenol can beoxidized to 4-ethyl-1,2-benzoquinone by tyrosinase and the detection isbased on the reduction of 4-ethyl-1,2-benzoquinone to4-ethyl-1,2-hydroquinone.

FIGS. 36A and 36B illustrate cyclic voltammetry responses of unmodifiedCNT electrode with and without p-ethylphenol (FIG. 36A) and TYR-modifiedelectrode in the presence and absence of p-ethylphenol (FIG. 36B).BQ-4-ethyl-1,2-benzoquinone and HQ-4-ethyl-1,2-hydroquinone.

FIGS. 37A and 37B illustrate detection of p-ethylphenol withtyrosinase-modified biosensor using cyclic voltammetry (CV) (FIG. 37A),and constant potential amperometry at potential of 0.13 V (FIG. 37B).Insert graphs display linear range of reliable detection, sensitivityand R² value.

FIG. 38 illustrates the stability of p-ethylphenol detection in 10, 25,50 and 100 μM p-ethylphenol solution on Day 1, 2, 4, 6, 8, 10 and 12.Percentage of current density retained is displayed (Insert).

FIG. 39 illustrates constant potential amperometry of interferencecompound: ethyl butanoate (EB), methyl hexanoate (MH), acetone (Ac),ethanol (Et), methyl butanoate (MB), 2-heptanone (HN) and 2-pentanone(PN) (Insert) and p-ethylguaiacol and control.

FIG. 40 illustrates Constant potential amperometry comparison ofsimulated sample and pure p-ethylphenol as control and sensitivitydetermination (Insert).

DETAILED DESCRIPTION

Before the present disclosure is described in greater detail, it is tobe understood that this disclosure is not limited to particularembodiments described, and as such may, of course, vary. It is also tobe understood that the terminology used herein is for the purpose ofdescribing particular embodiments only, and is not intended to belimiting.

Where a range of values is provided, it is understood that eachintervening value, to the tenth of the unit of the lower limit unlessthe context clearly dictates otherwise, between the upper and lowerlimit of that range and any other stated or intervening value in thatstated range, is encompassed within the disclosure. The upper and lowerlimits of these smaller ranges may independently be included in thesmaller ranges and are also encompassed within the disclosure, subjectto any specifically excluded limit in the stated range. Where the statedrange includes one or both of the limits, ranges excluding either orboth of those included limits are also included in the disclosure.

Unless defined otherwise, all technical and scientific terms used hereinhave the same meaning as commonly understood by one of ordinary skill inthe art to which this disclosure belongs. Although any methods andmaterials similar or equivalent to those described herein can also beused in the practice or testing of the present disclosure, the preferredmethods and materials are now described.

All publications and patents cited in this specification that areincorporated by reference, by notation in the application, areincorporated by reference to disclose and describe the methods and/ormaterials in connection with which the publications are cited. Thecitation of any publication is for its disclosure prior to the filingdate and should not be construed as an admission that the presentdisclosure is not entitled to antedate such publication by virtue ofprior disclosure. Further, the dates of publication provided could bedifferent from the actual publication dates that may need to beindependently confirmed.

As will be apparent to those of skill in the art upon reading thisdisclosure, each of the individual embodiments described and illustratedherein has discrete components and features which may be readilyseparated from or combined with the features of any of the other severalembodiments without departing from the scope or spirit of the presentdisclosure. Any recited method can be carried out in the order of eventsrecited or in any other order that is logically possible.

Embodiments of the present disclosure will employ, unless otherwiseindicated, techniques of molecular biology, microbiology, organicchemistry, biochemistry, botany, electrochemistry, and the like, whichare within the skill of the art. Such techniques are explained fully inthe literature.

It must be noted that, as used in the specification and the appendedembodiments, the singular forms “a,” “an,” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to “a support” includes a plurality of supports. Inthis specification and in the embodiments that follow, reference will bemade to a number of terms that shall be defined to have the followingmeanings unless a contrary intention is apparent.

As used herein, the following terms have the meanings ascribed to themunless specified otherwise. In this disclosure, “consisting essentiallyof” or “consists essentially” or the like, when applied to methods andcompositions encompassed by the present disclosure refers tocompositions like those disclosed herein, but which may containadditional structural groups, composition components or method steps (oranalogs or derivatives thereof as discussed above). Such additionalstructural groups, composition components or method steps, etc.,however, do not materially affect the basic and novel characteristic(s)of the compositions or methods, compared to those of the correspondingcompositions or methods disclosed herein. “Consisting essentially of” or“consists essentially” or the like, when applied to methods andcompositions encompassed by the present disclosure have the meaningascribed in U.S. Patent law and the term is open-ended, allowing for thepresence of more than that which is recited so long as basic or novelcharacteristics of that which is recited is not changed by the presenceof more than that which is recited, but excludes prior art embodiments.

Prior to describing the various embodiments, the following definitionsare provided and should be used unless otherwise indicated.

Definitions

In describing the disclosed subject matter, the following terminologywill be used in accordance with the definitions set forth below.

As used herein, “about,” “approximately,” and the like, when used inconnection with a numerical variable, generally refers to the value ofthe variable and to all values of the variable that are within theexperimental error (e.g., within the 95% confidence interval for themean) or within +/−10% of the indicated value, whichever is greater.

As used herein, “isolated” indicates removed or separated from thenative environment. An isolated peptide or protein (e.g., an enzyme)indicates the protein is separated from its natural environment.Isolated peptides or proteins are not necessarily purified.

The term “polypeptides” and “protein” include proteins, such as enzymes,and fragments thereof. Polypeptides are disclosed herein as amino acidresidue sequences. Those sequences are written left to right in thedirection from the amino to the carboxy terminus. In accordance withstandard nomenclature, amino acid residue sequences are denominated byeither a three letter or a single letter code as indicated as follows:Alanine (Ala, A), Arginine (Arg, R), Asparagine (Asn, N), Aspartic Acid(Asp, D), Cysteine (Cys, C), Glutamine (Gln, Q), Glutamic Acid (Glu, E),Glycine (Gly, G), Histidine (His, H), Isoleucine (Ile, I), Leucine (Leu,L), Lysine (Lys, K), Methionine (Met, M), Phenylalanine (Phe, F),Proline (Pro, P), Serine (Ser, S), Threonine (Thr, T), Tryptophan (Trp,W), Tyrosine (Tyr, Y), and Valine (Val, V).

Modifications and changes can be made in the structure of thepolypeptides of this disclosure and still obtain a molecule havingsimilar characteristics as the polypeptide (e.g., a conservative aminoacid substitution). For example, certain amino acids can be substitutedfor other amino acids in a sequence without appreciable loss ofactivity. Because it is the interactive capacity and nature of apolypeptide that defines that polypeptide's biological functionalactivity, certain amino acid sequence substitutions can be made in apolypeptide sequence and nevertheless obtain a polypeptide with likeproperties.

As used herein “functional variant” refers to a variant of a protein orpolypeptide (e.g., a variant of a plant enzyme) that can perform thesame functions or activities as the original protein or polypeptide,although not necessarily at the same level (e.g., the variant may haveenhanced, reduced or changed functionality, so long as it retains thebasic function).

“Identity,” as known in the art, is a relationship between two or morepolypeptide sequences, as determined by comparing the sequences. In theart, “identity” also refers to the degree of sequence relatednessbetween polypeptides as determined by the match between strings of suchsequences. “Identity” and “similarity” can be readily calculated byknown methods, including, but not limited to, those described inComputational Molecular Biology, Lesk, A. M., Ed., Oxford UniversityPress, New York, 1988; Biocomputing: Informatics and Genome Projects,Smith, D. W., Ed., Academic Press, New York, 1993; Computer Analysis ofSequence Data, Part I, Griffin, A. M., and Griffin, H. G., Eds., HumanaPress, New Jersey, 1994; Sequence Analysis in Molecular Biology, vonHeinje, G., Academic Press, 1987; and Sequence Analysis Primer,Gribskov, M. and Devereux, J., Eds., M Stockton Press, New York, 1991;and Carillo, H., and Lipman, D., SIAM J Applied Math., 48: 1073 (1988).

The terms “native,” “wild type”, or “unmodified” in reference to anorganism (e.g., plant or cell), polypeptide, protein or enzyme, are usedherein to provide a reference point for a variant/mutant of an organism,polypeptide, protein, or enzyme prior to its mutation and/ormodification (whether the mutation and/or modification occurrednaturally or by human design). Typically, the unmodified, native, orwild type organism, polypeptide, protein, or enzyme has an amino acidsequence that corresponds substantially or completely to the amino acidsequence of the polypeptide, protein, or enzyme as it generally occursnaturally.

As used in the present disclosure, two materials are in “electrochemicalcommunication” when electrons generated by a chemical reaction of onematerial (e.g., chemical reaction of a plant volatile compound with anenzyme) can be transferred to and/or accepted by the other material(e.g., another enzyme complex, a transducer material, and/or anelectrode) or vice versa.

As used in the present disclosure, a “transducer” describes a materialcapable of acting as an electronic transducer to transfer/deliverelectrons from one material/reaction to another. For instance, inembodiments of the electrochemical sensors of the present disclosure, asensor may include a transducer (e.g., a nanomaterial transducer) thatis in electrical communication between an enzyme active site and theelectrode. In embodiments, the electrode is formed from and/orfunctionalized with a transducer material and/or enzymes may be directlyor indirectly associated with the transducer material (e.g., byimmobilization on the nanomaterial transducer).

The term “bio-nanocomposite” refers to a material including bothbiologically derived material (e.g., proteins, enzymes, cells, otherbiological compounds) as well as a nanomaterial. In embodiments, abio-nanocomposite includes a nanomaterial such as metal or metal-oxidenanoparticles, carbon based nanoparticles (e.g., carbon nanotubes), orpolymeric materials, and a biological material, such as a protein (e.g.,an enzyme). In embodiments of a bio-nanocomposite of the presentdisclosure, the nanomaterial is a nanomaterial transducer material, andthe biological material is an enzyme that is directly or indirectlyassociated with and/or immobilized to and in electrical communicationwith the nanomaterial transducer material to form the bio-nanocomposite.

The term “detectable” refers to the ability to perceive or distinguish asignal over a background signal. “Detecting” refers to the act ofdetermining the presence of and recognizing a target or the occurrenceof an event by perceiving a signal that indicates the presence of atarget or occurrence of an event, where the signal is capable of beingperceived over a background signal.

Discussion

Embodiments of the present disclosure encompass methods and systems forelectrochemically sensing plant pathogen infection. Embodiments includeelectrochemical sensors capable of detecting plant volatiles and/orplant pathogen volatiles associated with plant pathogen infection,systems including the sensors, and methods of using the sensors todetect pathogen infection in plants, crops, or harvested plants and/orplant parts.

Plants and plant parts (leaf, stem, fruit, etc.) release uniquechemicals when they experience stress during a pest or pathogen attack(also referred to herein as “stress-induced plant volatilecompound(s)”). In some cases, the attacking pathogens also emit somecharacteristic chemicals indicative of infection (also referred toherein as “pathogen-emitted volatile compound(s)”). Detecting thesecharacteristic plant and/or pathogen chemicals at ultra low levels canbe used as a reliable way to detect pathogenic diseases andbiotic/abiotic stresses in crops. A reliable, easy-to-use sensingtechnology for detecting pathogen infections through chemical profilingat early stages of disease will alert the farmers to take preventive andcontrol measures to contain the infection and avoid massive crop damage.

The sensors, systems, and methods of the present disclosure includenovel bio-nanocomposite based electrochemical sensing technology forhighly selective and sensitive detection of chemicals resulting frompathogen infection of plants, such as, but not limited to, those inagricultural crops. Enzyme enhanced electrochemical sensing technologyoffers many advantages, such as, accuracy, specificity, rapid detection,non-invasiveness, field applicability, portability, robustness,selectivity, and ultra-low detection limits. The sensors of the presentdisclosure offer many advantages over conventionally used methods fordisease detection in the agricultural industry, which require molecularbiology methods and expensive spectroscopy methods that can only be usedto confirm the disease but cannot be used for early identification. Inthe bio-nanocomposite based electrochemical sensors of the presentdisclosure, the high sensitivity of the nanostructured materialscombined with high selectivity of enzymes will allow detection of thetarget volatile compounds in ultra-low quantities with minimalinterference from other volatile compounds. This can be achieved usingenzyme-functionalized nanomaterial composites as detection elements foramperometric transduction in electrochemical sensors. This approach hasthus far not been applied in the field of agricultural sensordevelopment.

Plants and pathogenic fungi release a complex mixture of volatileorganic compounds (VOCs), including, but not limited to, green leafvolatiles, flavor active alcohols, terpenes and other aromaticmetabolites. Since the chemical signature of the volatile mixture isoften unique to the type of crop or pathogen, detecting these volatilesat low levels can be used as a reliable method for detecting pathogenicdiseases and biotic/abiotic stresses in crops. The present disclosureprovides reliable, easy-to-use sensing technology for detecting pathogeninfections through volatile profiling at early stages of disease, whichwill allow time for implementation of preventive and control measures.Embodiments of the present disclosure also provide a portable sensorsystem (e.g., a ‘briefcase-biosensor’) to serve as a handy tool forindividual farmers to monitor their crops throughout the growth period.The provision of a portable system for real-time monitoring of cropconditions and early detection of pathogen infection will increaseagricultural productivity. Embodiments of the present disclosure alsoinclude a mobile smartphone application to correlate results of thebiosensor testing (e.g., a specific plant or crop volatile emissionprofile) with standard parameters for target volatile levels thatassociates types of pathogen infections with ranges of volatile typesand VOC levels. The sensors and systems of the present disclosure allowfarmers to evaluate their crop health independently without relying onoutside experts and to initiate measures for early intervention ofdetected infections. This will facilitate development of diseasemanagement strategies that reduce fungicide use, improve fungicideeffectiveness, minimize agricultural losses and environmental pollution,and save millions of dollars in lost productivity while improvingproduct quality.

As discussed above, most fungal pathogens release unique volatilesignatures upon attacking the host. In response, the plants and/orfruits also emit characteristic chemical signatures as a phytochemicaldefense. For example, Phytophthora sp. (a common oomycete) infectionresults in the release of octanone, ethyl phenol and ethyl guaiacol bythe fungus itself, and the release of methyl salicylate by the infectedcrop. Detection of the pathogen volatiles would indicate the existenceof fungal growth, and detection of the plant volatiles would indicatethe existence of plant stress. However, detection of both at the sametime could help relate the plant stress to the pathogen infection, whichcan deterministically confirm the onset of the disease. Therefore, insome embodiments, the electrochemical sensor of the present disclosureis capable of simultaneous detection of both the pathogen's activity andthe host's response to the infection, which can result in highlyselective detection of the disease at very early stages of infection.

As mentioned earlier, the VOCs released during an infection could comefrom both the pathogen and the plant parts (in response to theinfection). Plant tissues infected by various pathogens are known toproduce characteristic volatiles, stress-induced plant volatiles thatcan be used to differentiate infected and healthy plants. The compoundsare widely diverse and produced through various biosynthetic pathwaysincluding the octadecanoid pathway leading to fatty-acid derived greenleaf volatiles (GLVs), monoterpenes, diterpenes, sesquiterpenes,isothiocyanates and diverse groups of aromatic metabolites. Otherclasses of compounds also include benzene derivatives, indolederivatives, heterocyclic organic compounds, long chain aliphatichydrocarbons, aliphatic ketones, carboxylic acids, aldehydes andalcohols. For instance, as illustrated in FIG. 1, VOCs such as methylsalicylate, octan-one/-ol, ethyl phenol, ethyl guaiacol and green leafvolatiles are characteristic of the chemical signature in nearly a dozencrops upon infection by Fusarium spp, Phytophthora spp, and Sclerotiumspp. While some are released from the infected fruits, others arereleased from the stem and leaves. For example, strawberries infectedwith Phytophthora cactorum were found to release 4-ethyl phenol and4-ethyl guaiacol. Methyl salicylate and methyl jasmonate are often foundto be common and distinct odors of infected plants. Plants release highlevels of methyl salicylate especially during pathogen or pestinfestations through the Shikimate biosynthesis pathway. In controlledlaboratory tests, significant levels of methyl salicylate were releasedby pepper plants when infected by Phytophthora capsici. Studies showthat unhealthy soy pods also release methyl salicylate. Moreover, unlikeGLVs, methyl salicylate is an allelochemical and is released not only atthe infection site, but throughout the plant through a systematicresponse. Octanone and/or octanol are found in VOCs collected from thesite of most fungal infections. GLVs such as hexenol, hexenal, hexylacetate, hexenyl acetate, and terpenes are also released in excess,indicative of a stressed plant.

Basic electrochemical detection of some common GLVs such ascis-3-hexenol, hexyl acetate and hexenyl acetate was achieved usingamperometric methods as described in Umasankar, Y., et al., Analyst2012, which is incorporated by reference herein. Detection limits as lowas 28 nM (˜0.5 ppb), a value 138 times lower than the human odorthreshold limit (OTL) of 3.89 μM, were achieved; however,bio-recognition elements (e.g., an enzyme specific for the targetvolatile compound) was not used for specific detection.

Amperometric detection of methyl salicylate and ethyl guaiacol wasperformed using gold and metal oxide nanoparticles as both detectionelements and transducers on electrodes as described in Umasankar, Y. &Ramasamy, R. P., Analyst 2013 and Costello, et al., Sensors andActuators B 1999, which are incorporated herein by reference. In thefirst study, the use of gold nanoparticle (AuNP) modified electrode formethyl salicylate detection resulted in a sensitivity increase by ˜35fold and widened range of detection over unmodified carbon electrodes.In a second study, described in more detail in Example 2, below, metaloxide nanoparticles such as TiO₂ and SnO₂ were used to functionalize anelectrode for detection of ethyl guaiacol, which is a typical VOC ofinfected berries and grapes. Both metal oxides exhibited fairly goodsensitivity towards ethyl guaiacol of ˜0.23 mA/mM·cm² and ultra-lowdetection limits around 60 nM (˜1.1 ppb), 46 fold lower than OTL of 2.78μM, but specific recognition elements (e.g., enzymes) were not providedfor specificity.

While these nanomaterial-modified electrodes provide high surface areafor enhanced sensitivity, they do not offer high selectivity (i.e.,specificity) towards the target compound for detection. Since multiplevolatile compounds can contribute to the electrochemical signals in thesame potential window, it would be challenging to identify/recognize,let alone confirm the presence of a particular compound in a mixture byusing just the nanomaterials. The electrochemical sensors of the presentdisclosure overcome this difficulty by using enzymes as bio-recognitionelements on the sensor electrode platform.

Enzymes such as tyrosinase (TYR), laccase (Lc), bilirubin oxidase (BRO)and horseradish peroxidase (HRP) can be used to catalyze biochemicalreactions involving oxygen or peroxide. Moreover, being phenol oxidases,these enzymes are also highly specific towards the reversible redoxreactions of a wide number of phenolics and their corresponding quinonecounterparts. The sensors and methods of the present disclosuredemonstrate that when combined with nanomaterials such as carbonnanotubes (CNTs), graphene, gold, and metal oxide nanoparticles, theenzymes exhibit excellent bio-electrochemical activity towards thetarget analytes favoring fast and reliable amperometric detection. Forexample, as shown in FIG. 2A, an enhanced signal for ethyl phenol andethyl guaiacol detection was observed when HRP was used as thebio-recognition element in a CNT-polymer matrix modified electrode. AlsoTYR and Lc can be used to electrochemically detect phenolic compoundswith high specificity based on their quinone products (see Costello 1999and Umasankar, Y. and Ramasamy, R. P., ECS Transactions 2013, both ofwhich are incorporated by reference herein). Also, as described in moredetail in Example 1, below, a highly selective detection of methylsalicylate was achieved using a bi-enzyme system using a combination ofsalicylate hydroxylase and tyrosinase (our NSF project) or in a secondapproach using a combination of alcohol oxidase (AO) and horseradishperoxidase, respectively. FIG. 2B also shows the voltammetric responseof a bi-enzyme (AO) and (HRP) functionalized CNT electrode towardsmethyl salicylate in alkaline buffer with a sensitivity of 0.285mA/mM·cm² and detection limit as low as 980 nM (˜17 ppb), 2.3 timeslower than OTL. With this capability (2 mL cell), a typical earlyrelease rate from the plants at 283 ng/hr/plant, would produce enoughsample volume for detection within 1.05 hours, much earlier than visualsymptoms occur. Currently no other method can achieve this withoutrequiring expert analysis and expensive instrumentation.

Thus, the present disclosure provides electrochemical sensors, plantvolatile detection systems, and methods for monitoring conditions of aplant or crop that utilize a bio-nanocomposite functionalized electrodefor detection of volatile compounds indicative of plant stress and/orpathogen infection. In embodiments, the electrochemical sensors,detection systems, and methods of the present disclosure not only detectbut can also quantify target volatile compounds. The bio-nanocompositedetection elements include a nanomaterial transducer material forenhanced sensitivity and enzymes capable of specific reaction with atarget volatile for enhanced specificity. The volatile detectionelectrodes of the present disclosure are also configured to provideoptimal electrochemical communication between the bio-nanocomposite andthe electrode.

Volatile Detection Electrode

Embodiments of an electrochemical sensor of the present disclosureinclude a volatile detection electrode made of an electrode substrateand a bio-nanocomposite detection element on a surface of and/or inelectrical communication with the electrode substrate. Thebio-nanocomposite detection element includes a nanomaterial transducermaterial and at least one enzyme capable of specific reaction with atarget volatile compound or its hydrolysis product. The nanomaterialtransducer material act as nanomaterial supports for the enzymes as wellas electronic transducers. This offers benefits including, but notlimited to, enhanced surface area (providing enhancement ofsensitivity), reliable support for enzyme attachment, and enhancedspecificity due to high enzyme loading. In embodiments, the enzyme(s)are immobilized to the nanomaterial transducer material, and reactionbetween the enzyme(s) and the target volatile compound generates anelectrical signal that is detected at the electrode. Detection of theelectrical signal indicates the presence of the target volatilecompound. The target volatile compound can be a stress-induced plantvolatile compound and/or a target pathogen-emitted volatile compound. Inembodiments, the electrochemical sensors of the present disclosure candetect both a stress-induced plant volatile and a pathogen-emittedvolatile compound associated with a pathogen that induces thestress-induced plant volatile in the plant.

The electrode substrate can be any substrate capable of beingfunctionalized with a nanomaterial transducer material and/or thebio-nanocomposite detection element. Examples of electrode materialsinclude carbon, gold, platinum, silver, ruthenium, palladium, rhodium,osmium, iridium, or the like. In some embodiments, the nanomaterialtransducer material can serve as the electrode and the transducermaterial. In such embodiments where the nanomaterial serves as theelectrode, the substrate does not have to be electrically conductive andcan be made of materials including, without limitation, ceramicmaterials, such as oxides (e.g., silica, fused silica, amorphous silica,fused amorphous silica, sapphire, or the like), nitrides (e.g., siliconnitride, boron nitride, or the like), carbides, oxycarbides,oxynitrides, or the like; polymeric materials (e.g., epoxies, phenolicpapers, polyesters, or the like); fiberglass; or the like. Inembodiments, the electrode substrate can be carbon, such as, but notlimited to a modified screen printed carbon electrode (SPCE).

In embodiments, the nanomaterial transducer material is selected fromcarbon nanoparticles (e.g., multiwalled carbon nanotubes (MWCNTs)),metal nanoparticles, and metal oxide based nanomaterials. Inembodiments, metal nanoparticles can include, but are not limited to,gold, silver, and/or platinum nanoparticles. In embodiments, thenanomaterial transducer material includes multiwalled carbon nanotubes.

Metal oxide (MO_(x)) nanomaterials are inexpensive alternatives toprecious metals (Au, Ag or Pt), and offer characteristics important forelectrochemical sensor applications at a fraction of the cost. Metaloxides can act as good catalysts for dehydrogenation and/ordecomposition of VOCs such as aliphatic alcohols, ketones, acetic acid,and the like. Also, by varying the shape and size of MO_(x), one cancontrol their chemical adsorption properties. Third, intrinsicallyn-type semi-conducting MO_(x) such as TiO₂, SnO₂, and ZnO can be usedfor amperometric signal generation even in aqueous environments. Inembodiments, metal oxide nanoparticles can include, but are not limitedto, TiO₂, SnO₂, ZnO, and indium-tin oxide (ITO). In embodiments, thenanoparticles can be a specific shape, such as nanorods. In embodiments,the nanomaterial transducer material can be ITO nanoparticles, such asITO nanorods. Some exemplary nanomaterial transducer materials for theelectrochemical sensors of the present disclosure are discussed ingreater detail below in conjunction with exemplary nanomaterial enzymecombinations as well as in the Examples below.

The enzymes for the sensors of the present disclosure include anyenzymes capable of specific reaction with a target stress-induced plantvolatile compound or a target pathogen-emitted volatile compound or thehydrolysis product of any of these compounds. In embodiments the enzymeis capable of specific reaction with a target stress-induce plantvolatile compound or target pathogen-emitted volatile compound, but inother embodiments, the volatile compound is first hydrolyzed (e.g., byexposure to other chemicals in the sensor system or environment) andthen one or more immobilized enzymes reacts with the hydrolysis product.In embodiments, there are more than one enzyme where each enzyme reactswith a different compound in a cascade of reactions beginning with thetarget volatile compound. In embodiments, target stress-induced plantvolatile compounds include, but are not limited to, methyl salicylate,ethyl phenol, ethyl guaiacol, octanone, octanol, green leaf volatilecompounds (e.g., hexenol, hexenal, hexyl acetate, hexenyl acetate, andterpenes), and derivatives of these volatile compounds. Exemplary plantpathogens for detection in the present disclosure include, but are notlimited to, Fusarium species, a Phytophthora species, and a Sclerotiumspecies. In embodiments, target pathogen-emitted volatile compoundsinclude volatile compounds emitted from one or more of the above-listedplant pathogens. In embodiments, target pathogen-emitted volatilecompounds include, but are not limited to ethyl phenol, ethyl guaiacol,octanone, and combinations of these volatile compounds. Additionaldetails regarding some of the above-listed volatile compounds areprovided below.

Exemplary enzymes capable of reaction with one or more of theabove-listed volatile compounds or their hydrolysis products include,but are not limited to: tyrosinase (TYR), laccase (Lc), bilirubinoxidase (BRO), horseradish peroxidase (HRP), salicylate hydroxylase,alcohol oxidase (AO), alcohol dehydrogenase (ADH), tannase, esterase,and combinations of more than one of these enzymes. In some embodiments,one or more enzymes can be included in a volatile detection electrode,where the enzymes interact with the target volatile compound to producea cascade of reactions, the product of which is detected by the sensor.For instance, in an embodiment of a multi-enzyme system, a bi-enzymesystem may be used, where at least one enzyme reacts with the targetvolatile compound or its hydrolysis product to produce a first reactionproduct, and a second enzyme reacts with the first reaction product toproduce a second reaction product, where the second reaction product orthe second reaction produces an electrical signal capable of beingdetected by the electrode. In embodiments, a tri-enzyme system may alsobe employed, where at least one enzyme reacts with the target volatilecompound producing a cascade of reactions involving the other enzymes ofthe system, where at least one of the reactions (e.g., the finalreaction in the cascade) produces an electrical signal capable of beingdetected by the electrode. Although bi-enzyme and tri-enzyme systems aredescribed in greater detail herein, variations on such multi-enzymesystems are contemplated within the scope of the present disclosure.Additional details with respect to some of the above-listed enzymes andenzyme systems are provided in the discussion below.

In embodiments, the electrochemical sensor of the present disclosure canspecifically detect two or more different target volatile compounds. Inembodiments, the electrochemical sensor includes two or more volatiledetection electrodes, where at least one volatile detection electrodedetects a different target volatile compound than at least one othervolatile detection electrode. In other embodiments, a single volatiledetection electrode may be configured with different areas for detectionof different volatile compounds.

In some embodiments, the electrochemical sensor includes two or morevolatile detection electrodes, where a first volatile detectionelectrode detects a target plant pathogen-emitted volatile compound anda second volatile detection electrode detects a target stress-inducedplant volatile associated with infection by a plant pathogen that emitsthe target plant pathogen-emitted volatile compound. In this manner, thesensor can detect both the pathogen as well as that the plant is instress from the pathogen, providing further verification of infection.For instance, in some embodiments, the stress-induced plant volatile ismethyl salicylate and the target plant pathogen-emitted volatilecompound is selected from the group of VOCs including, but not limitedto: octanone, ethyl phenol, and ethyl guaiacol. Other embodimentsinclude other combinations of plant pathogen-emitted volatile compoundsand stress-induced plant volatile compounds.

Functionalizing nanomaterials with enzymes to provide thebio-nanocomposite detection element of the biosensor involves anintimate attachment between the two, so that an electrical communicationcan be established between the nanomaterial transducer and enzyme activesite. The enzyme active site is often buried deep inside the insulatingpolypeptide matrix and may be unavailable for direct communication withthe electrode. To overcome this, in embodiments, a “redox shuttle” or“mediator” could be used to facilitate the electron transfer, as shownin FIG. 3A. In embodiments of the present disclosure, nanomaterialsupports and immobilization methods are selected that favor directelectrical communication (FIG. 3B).

Various approaches known to those of skill in the art for enzymeimmobilization to carbon, metal, and metal oxide based nanomaterials canbe employed for the sensors of the present disclosure. Immobilizationcan be by direct or indirect linking so long as it provides forelectrochemical communication between the enzyme and the nanomaterial.For example, a non-covalent sidewall functionalization can be effectivefor direct electrical communication between metalloenzymes and carbonnanotube/ graphenes (for example as described in Ramasamy, et al., ChemCommun 2010, Parimi, et al., Acs Catal 2012, and Calkins, et al., EnergEnviron Sci 2013, which are hereby incorporated by reference herein).Dendrimer based covalent strategies can be used in embodiments to bindenzymes onto gold nanoparticles (for example as described in Umasankarand Ramasamy, ESC Transactions 2013 (incorporated by reference above)).In other embodiments, a two-step immobilization procedure can be used toimmobilize multiple enzymes onto metal oxide nanostructures, resultingin high bio-electrocatalytic activity.

In embodiments, enzyme immobilization is achieved using an approach inwhich the nanomaterial transducer material (such as, but not limited to,ITO nanoparticles) is functionalized with a silane cross-linker havingterminal amine groups (e.g., APTES) as illustrated in FIG. 5. Then anamine-amine crosslinker (e.g., 1,5-difluro-2,4-dinitrobenzene) can beused to establish protein attachment linkers on the surface to which theenzymes will be covalently attached through imide-bonds. The length ofcross-linker can be adjusted to accommodate different enzyme sizes. Inembodiments, such as with a multi-enzyme system, two or morecrosslinkers of varying lengths can be employed to accommodate two ormore layers of enzymes.

In other embodiments, such as when the nanomaterial transducer materialis MWCNTs, the nanomaterial is non-covalently functionalized with atethering agent, such as, but not limited to, 1-pyrene butanoic acidsuccinimidyl ester (PBSE), as illustrated in FIG. 6. In embodiments, thetethering agent can be other heterobifunctional tethering agents of thePBSE-type, such as, but not limited to4,4′-[(8,16-dihydro-8,16-dioxodibenzo[a,j]perylene-2,10-diyl)dioxy]dibutyric acid di(N-succinimidyl ester) (DPPSE). The NHS-ester groups ofthe molecular tethers can be covalently linked to amine groups of theone or more enzymes to be used for specific detection. In embodiments,such as illustrated in FIGS. 7A and 7B, the surface of carbon nanotubesas the transducer material is modified withpoly-vinylalcohol-N-methyl-4(4′-formylstyryl)pyridinium-methosulfateacetal (PVA-SbQ) polymer, a polymer that enhancesconductivity and facilitates charge transfer in the bio-nanocompositesvia electrostatic interactions. In addition, this photo-switchablepolymer can also be activated by UV light to cross-link the SbQ sidechains, thereby entrapping the enzyme and nanomaterials resulting inhigh enzyme stability over time.

Example Volatile Detection Enzymes

Some exemplary enzyme/volatile compound detection combinations aredescribed in additional detail in the present section, but one of skillin the art will understand that these are not intended to be limitingexamples and other combinations and variations are possible.

Methyl Salicylate: In embodiments of the electrochemical sensor of thepresent disclosure for methyl salicylate detection, a multi-enzymesystem can be used for detection. In an embodiment, a bi-enzyme systemis used for amperometric detection as shown in FIG. 4A and described inExample 1 below. The detection is based on cascade enzyme reactions: (i)room temperature hydrolysis of methyl salicylate to produce methanol;(ii) the enzymatic conversion of oxygen and methanol to hydrogenperoxide and H₂O₂ by alcohol oxidase (AO) enzyme; and, finally, (iii)the enzymatic electrochemical reduction of H₂O₂ to water usinghorseradish peroxidase (HRP). The electrons used for the HRP reactionwill generate an amperometric signal that can be correlated to theconcentration (quantity) of methyl salicylate in the analyte. Thismethod of detection is highly selective and only happens when bothenzymes are present, with little or no interference from other reactionproducts. Also it provides more reliable detection over methods based onsalicylate hydroxylase and methanol dehydrogenase enzymes.

Another embodiment of a bi-enzyme system for detection of methylsalicylate is described below in Example 3. In an embodiment, thebi-enzyme system includes salicylate hydroxylase (SH) and tyrosinase(TYR). The system is based on the following cascade of reactions: (i)hydrolysis of methyl salicylate to salicylate; (ii) the enzymaticconversion of salicylate to catechol by SH; and, finally, (iii) theenzymatic oxidation of catechol to 1,2-benzoquinone by TYR. Thereduction current of the 1,2-benzoquinone produced by the final reactioncan be detected and measured at the electrode.

In another embodiment, a tri-enzyme system, such as described in greaterdetail in Example 4, below, can be used for methyl salicylate detection.In embodiments, the tri-enzyme system employs another enzyme at thebeginning of the cascade to catalyze the initial hydrolysis of methylsalicylate, for a total of 3 enzymes in the cascade. Thus, the threeenzyme system employs a first enzyme for the hydrolysis of methylsalicylate and then two additional enzymes, such as described above. Inembodiments, the first enzyme in the tri-enzyme system can be tannase,esterase, or other enzymes capable of hydrolysis of methyl salicylate tosalicylate and methanol. The two other enzymes can be an enzyme pairsuch as, but not limited to: SH/TYR or AO/HRP, as described above.

Ethyl-phenol and Ethyl-guaiacol: In embodiments of the electrochemicalsensor of the present disclosure for ethyl phenol and ethyl guaiacoldetection, phenol oxidases that possess good selectivity towards eitherof these compounds can be used. Both tyrosinase (TYR) and horseradishperoxidase (HRP) enzymes exhibit high electrochemical activity towardsethyl phenol and ethyl guaiacol oxidations (see FIGS. 2A-2B). As shownin FIG. 4B, in the absence of O₂ and H₂O₂, the enzymes TYR or HRPoxidize phenolic compounds and deliver the electrons to the electrode,which can be captured as an amperometric signal for the analytedetection.

Non-enzyme based detection of ethyl-guaiacol is described in Example 2below. Such embodiments can be modified with enzyme systems as describedherein for improved specificity.

An embodiment of an enzyme based electrochemical sensor for detection ofethyl phenol is described in greater detail in Example 5, below. Anembodiment of an electrochemical sensor of the present disclosure forp-ethylphenol detection includes tyrosine as the enzyme in electricalcommunication with the nanomaterial transducer material (such as MWCNTs,metal oxide nanoparticles, and the like as described above). In suchembodiments, tyrosinase enzymatically oxidizes p-ethylphenol to4-ethyl-1,2-benzoquinone. The reduction of 4-ethyl-1,2-benzoquinone to4-ethyl-1,2-hydroquinone is detected at the electrode. Additionaldescription of this embodiment is provided in Example 5, below.

Octanone: This long chain aliphatic ketone is typically hard to detectselectively using conventional nanomaterials. However, in embodiments ofthe electrochemical sensor of the present disclosure for detectingoctanone, upon functionalizing the nanomaterials with alcoholdehydrogenase (ADH) enzymes, the compound and its corresponding reducedform, octanol can be selectively detected based on the NADH oxidationcurrents, as NADH progressively gets consumed in the reaction, as shownin FIG. 4C. Regeneration of cofactors such as NADH is not critical, asthe sensor electrode strips are not meant to be re-used. This enzyme hasbeen used before for demonstrating ketone reduction, but not forelectrochemical sensing.

A challenge in dealing with enzymes is to create a compatibleenvironment for both the reactants (target compounds) and the enzymesused for the reactions. Ideally the VOCs are solubilized in desiredconcentrations. Most of the target VOCs are sufficiently soluble inaqueous buffer at enzyme relevant pH. However, in case of solubilityissues (possibly with octanone), biocompatible, water soluble, ionicliquid electrolytes (e.g AMMOENG™ 101) can be used as prescribed in theliterature, for example Kohlmann, C. et al., Environ Entomol 2003; vanRantwijk, F. and Sheldon, R.A., Chem Rev 2007; and roosen, C. et al.,Appl Microbiol Biot. 2008, which are incorporated by reference herein.

Green leaf volatiles: Various routes exist for electrochemical detectionof common green leaf volatiles (GLV) such as hexenol, hexanone and hexylacetate using electrochemical biosensors (see, Umasankar, Y., et al.,Electroanalytical studies on green leaf volatiles for potential sensordevelopment. Analyst 2012, 137, 3138-3145, incorporated by referenceherein). Such systems can be integrated into the enzyme basedelectrochemical biosensors of the present disclosure. GLV have also beenevaluated in the examples below as potential interfering compounds forthe above-described biosensors, as GLV are commonly released under allconditions, but at varying concentrations.

Example Enzyme-Nanomaterial Composite

Some exemplary enzyme-nanomaterial composite combinations are describedin additional detail in the present section, but these are not intendedto be limiting examples, and other combinations and variations arepossible.

Due to their low active site loading per unit area, for the sensors ofthe present disclosure, enzymes are immobilized on nanomaterial supportsthat also act as electronic transducers. This forms a bio-nanocompositedetection element that provides multiple benefits (i) enhanced surfacearea (sensitivity enhancement); (ii) reliable support for enzymeattachment and (iii) enhanced specificity due to high enzyme loading.However, the choice of nanomaterials depends on their properties and thetype of enzyme immobilization methods. Some non-limiting examplesdiscussed in greater detail here include indium-tin oxide (ITO) andcarbon nanotubes, described briefly above, both of which providedesirable properties for the target reactions and enzymes.

Indium Tin Oxide Nanostructures

Metal oxide (MOx) nanomaterials are inexpensive alternative to preciousmetals (Au, Ag or Pt), and offer many characteristics desirable forelectrochemical sensor applications, at a fraction of the cost. Metaloxides have been reported to act as good catalysts for dehydrogenationand/or decomposition of VOCs such as aliphatic alcohols, ketones, aceticacid, etc. By varying their shape and size, one can control theirchemical adsorption properties. Intrinsically n-type semi-conducting MOxsuch as TiO₂, SnO₂, ZnO can be used for amperometric signal generationeven in aqueous environment as demonstrated in Example 2, below.Finally, the surface of MOx nanoparticles can be functionalized withenzymes for bioelectrochemical reactions, such as illustrated in FIG. 5Aand as demonstrated in Zhou, Y., et al., 225th Electrochemical SocietyMeeting, Orlando, 2014 and Zhou, Y. et al., Acs Catal 2014, incorporatedby reference above.

Embodiments of two architectures of highly conducting indium-tin oxide(ITO) nanomaterials can be used as enzyme supports. One type iscommercially available ITO nanoparticles in different sizes that can bedeposited on the strip electrodes. Another type is ITO nanorod arrays,which can be directly grown on the surface of ITO coated conductingglass slides or strips electrodes using oblique angle physical vapordeposition (PVD) process as described in Wolcott, A. et al., Small 2009.The length, diameter and spacing (density) of the ITO nanorods can becontrolled by varying the PVD parameters.

The ITO nanoparticles and nanorods can then be subject to enzymeimmobilization. In embodiments, an immobilization approach is used asdescribed in Zhou, 225^(th) Electrochemical Society Meeting 2014 andZhou ACS Catal 2014, incorporated by reference above. Briefly, as shownin FIG. 5B, first the ITO nanostructures (particles, rods, or spheres)are functionalized with a silane cross-linker terminating with aminegroups (e.g., APTES). Next an amine-amine crosslinker (e.g.,1,5-difluro-2,4-dinitrobenzene) is used to establish protein attachmentlinkers on the surface to which the enzymes will be covalently attachedthrough imide-bonds. The length of cross-linker can be adjusted toaccommodate different enzyme sizes. The choice of enzyme will depend onthe type of target compound as discussed herein. For bi-enzyme systems,in embodiments two crosslinkers of varying lengths can be used toaccommodate two layers of enzymes or co-immobilize them if pH conditionspermit.

Carbon Nanotubes

In embodiments multi-walled carbon nanotubes (MWCNTs) can be used bothas stand-alone material and as enzyme immobilization supports for sensorelectrode development. CNT have high affinity towards aromatic moieties,present in VOCs. For linking enzymes and CNTs to establish directelectrical communication, in some embodiments a molecular tetheringmethod can be used, such as described in Ramasamy, et al., Chem Commun2010, incorporated by reference above. In this method, MWCNT can benon-covalently functionalized with 1-pyrene butanoic acid succinimidylester (PBSE), such as illustrated in FIG. 6, or other similar tetheringagents such as N-1(1-pyrenyl maleimide),4,4′-[(8,16-dihydro-8,16-dioxodibenzo[a,j]perylene-2,10-diyl)dioxy]dibutyric acid di(N-succinimidyl ester) (DPPSE) and other PBSE-typeheterobifunctional tethering agents (see Atanassov, et al., 2014,incorporated by reference herein). Then, the NHS-ester groups of themolecular tethers will be covalently linked to the amine groups of theenzyme. Since the diameters (curvature) and chirality plays an importantrole in this type of immobilization, they will be varied as needed usingcommercially available nanotubes. This method suits both AO and HRP fordeveloping bi-enzyme systems.

In embodiments, such as illustrated in FIGS. 7A and 7B, the surface ofCNT was modified with poly-vinylalcohol-N-methyl-4(4′-formylstyryl)pyridinium-methosulfateacetal (PVA-SbQ) polymer (see Nam, et al., OrgElectron 2009, which is hereby incorporated by reference herein), whichwas found to enhance conductivity and facilitates charge transfer in thebio-nanocomposites via electrostatic interactions. In addition, thisphoto-switchable polymer can also be activated by UV light to cross-linkthe SbQ side chains, thereby entrapping the enzyme and nanomaterialsresulting in high enzyme stability over time. Magnified images of enzymemodified and unmodified CNT surfaces are illustrated in FIG. 8.

Systems

In embodiments, the electrochemical sensors of the present disclosureare part of a detection system. In embodiments, the volatile detectionelectrode is a working electrode of an electrochemical cell, such as astandard 3 electrode cell. In embodiments, the electrochemical cell alsoincludes a counter electrode and reference electrode in electrochemicalcommunication with the working electrode and a potentiostat to supply anelectrical current to the electrochemical cell and monitor changes inthe electric current generated at the working electrode. In embodiments,the changes in the electric current in the electrochemical cell arerecorded as a cyclic voltammogram, differential pulse voltammogram orsome other form of current response to an applied potential or voltage.

In embodiments, the electrochemical sensor is part of a plant volatiledetection system. In embodiments, plant volatile detection systems ofthe present disclosure include a volatile collection reservoir adaptedto collect volatile compounds emitted from a plant, an electrochemicalsensor, and a signal processing mechanism. In embodiments, theelectrochemical sensor includes an electrochemical cell having avolatile detection electrode as described above (the working electrode),counter and reference electrodes, and a potentiostat to supply anelectric current to the electrochemical cell and monitor changes in theelectric current produced at the working electrode. The volatiledetection electrode is in fluid communication with the volatilecollection reservoir such that volatile compounds collected in thereservoir can be transferred to a detection surface of the volatiledetection electrode, and the counter and reference electrodes are inelectrochemical communication with the volatile detection electrode. Inembodiments, the signal processing mechanism in operative communicationwith one or more elements of the electrochemical sensor, and the signalprocessing mechanism has data transfer and evaluation software protocolsconfigured to transform raw data from the electrochemical sensor intodiagnostic information regarding the presence or absence or levels ofthe plant volatile compound.

In embodiments, the signal processing mechanism can be, but is notlimited to, a personal computer, a mainframe, a portable computer, apersonal data assistant, a smart phone, and a tablet computer, or acombination thereof. In embodiments, the plant volatile detection systemis portable and adapted for sampling volatiles in a field environment.Other embodiments include a smart phone application configured toreceive information from the signal processing mechanism and transformthe information into alerts, recommendations, or both for a user.

In general, detection systems of the present disclosure includeadditional instrumentation for the system (e.g., signal processingcircuitry, a reference electrode, a counter electrode, a potentiostat,and/or an electrochemical workstation), and a signal processingmechanism (e.g., a personal computer, mainframe, portable computer,personal data assistant, or the like), each of which could be inoperative communication with one or more of the other components. Forinstance, in embodiments, the electrochemical sensor systems anddetection systems of the present disclosure may include or may beintegrated with at least one of the following: a reference electrode anda counter/auxiliary electrode; one reference electrode and onecounter/auxiliary electrode for each volatile detection electrode in asystem; an electrochemical workstation; a signal processing mechanism,wherein the signal processing mechanism comprises data transfer andevaluation software protocols configured to transform raw data intodiagnostic information; a temperature control mechanism; or a fluidcontrol mechanism.

By way of example, the volatile detection system can be configured suchthat the volatile detection electrode (working electrode) is coupled tocounter and reference electrodes and an electrochemical workstation thatprovides a current or voltage source to the electrodes to effect a flowof electrons to the electrochemical cell that is monitored and measuredat the workstation by a computer, which reports and records thevoltammetric current. The voltammetric current, and changes therein, canbe recorded as a cyclic voltammogram. The computer system can includedata transfer and evaluation protocol capable of transforming raw datafrom the volatile detection electrode into information regarding thepresence and/or absence of a target analyte. The computer can also becapable of providing diagnostic information regarding the targetanalyte. In certain situations, the computer is a portable personalcomputer that includes data transfer and evaluation software capable ofstoring and analyzing the recorded signals. Under these circumstances,the biosensor instrument can provide a diagnostic tool that itself isportable and is powered from the laptop computer.

The electrochemical sensors of the present disclosure are capable ofproviding a specific electrochemical excitation signal that is optimizedto yield the maximum diagnostic value. These systems thus can representa complete diagnostic package with the capability to aid rapid analysisby a person who has minimum technical training. In exemplaryembodiments, the raw electrochemical output from the electrochemicalsensor of the present disclosure is collected and transferred to thememory of a computer, which includes a pattern recognition evaluationprogram that can be “trained” to identify a specific binding event andalso the degree of the matching between the capture molecule and thetarget analyte (e.g., stress-induced plant volatile and/or pathogenemitted volatile), and thus recognize the signature of a particularbinding event for which it was “trained”. Such a detection systemprovides a complete diagnostic package whose purpose is to aid rapidscreening, detection, and analysis of a target analyte, withoutelaborate preparation, by a person who has minimum technical trainingand to enable portability of such a system, bringing heretoforeunavailable diagnostic and monitoring capabilities to large and/orremote areas.

The various electrodes can be in operative communication with anelectrochemical workstation that provides a current or voltage source tothe three electrode cell. This provides a flow of electrons to thethree-electrode cell(s) that is monitored and measured at theworkstation by a signal processing mechanism, which reports and recordsthe voltammetric current. The voltammetric current, and changes therein,can be recorded as a cyclic voltammogram. The workstation may provide avoltage source to the electrode and measure a current, but it is alsocapable of working in reverse providing a current source and measuring avoltage. Either set-up is acceptable for operating the biosensorinstruments of the present disclosure.

The signal processing mechanism can be a personal computer, mainframe,portable computer, personal data assistant, or the like. The signalprocessing mechanism can include data transfer and evaluation protocolcapable of transforming raw data from the biosensor array intoinformation regarding the presence, absence, and the extent of theinteraction of a target volatile compound(s). The signal processingmechanism can also be capable of providing diagnostic informationregarding the target volatile(s).

Generally, the solid state electronics, including, for example, apotentiostat circuit connected to working and reference electrodes, asdescribed above for performing electrochemical measurements, areexternal to the volatile detection electrode (and any printed circuitboard package associated with the volatile detection electrode to enableconnection to other elements of an electrochemical cell potentiostatcircuitry, etc.). Notwithstanding this, the volatile detectionelectrode, reference and counter/auxiliary electrodes, electrochemicalworkstation, and signal processing mechanism can be arranged in avariety of configurations, when in combination with other componentsthat are known to those of skill in the art.

Again, the volatile detection electrode can also include signalprocessing circuitry, as discussed above. The electrode is thencontacted with a sample to be analyzed (e.g., in sufficient contact withthe sample for a target volatile compound contained in the sample tointeract with the enzyme(s) on the volatile detection electrode), andthe system is interrogated using standard electrochemical techniques. Asdiscussed above, the electrochemical sensor/detection system includes acurrent source to provide a flow of electrons to drive theelectrochemical processes at the volatile detection electrode and asignal processing mechanism for detecting and reporting any change atthe electrode. As discussed above, some embodiments of the volatiledetection system also include a data analysis component (e.g., dataanalysis software on a computer system coupled to the biosensor arraydescribed above) for storing and evaluating the electrochemical signalproduced by the biosensor-chip array.

Those skilled in the art to which this disclosure pertains will alsoappreciate that the sensing portion of the biosensor instrument (e.g.,volatile detection electrode) can be reusable. The sensor can be washedand reused to detect the same volatile compound(s) in a differentsample. Those of skill in the art will also understand that theelectrochemical volatile sensor of the present disclosure, prepared inan array format, can be adapted to detect many different volatilecompounds and used for high throughput applications.

In embodiments, the plant volatile detection system of the presentdisclosure can be portable and adapted for sampling volatiles in a fieldenvironment. In some such embodiments, the signal processing mechanismmay be a portable personal computer, such as a laptop, tablet, or thelike. In embodiments, the plant volatile detection system of the presentdisclosure can also include or be configured to interact with a smartphone application configured to receive information from the signalprocessing mechanism and transform the information into alerts,recommendations, or both for a user. Such portable systems can be usefulfor field application, so that farmers or other professionals ortechnicians can carry the equipment to the field for testing, ratherthan bringing a sample back to a laboratory or sending it off fortesting.

Methods for monitoring plants

The present disclosure also includes methods of using the sensors andsystems of the present disclosure to monitor the condition plants,crops, harvested plants and plant parts (e.g., fruits, vegetables,etc.). In embodiments, the methods include monitoring for infection by aplant pathogen, such as but not limited to a fungus, insect, orbacterial organism that produces volatile compounds and/or induces theinfected plant to produce certain stress-induced volatile compounds.

In embodiments, methods of the present disclosure for monitoring acondition of a plant or crop of plants include periodically samplingvolatile emissions from the plant or one or more crop plants using theelectrochemical sensors and/or plant volatile detection systems of thepresent disclosure and analyzing the information provided by the signalprocessing mechanism, where the presence or amount of one or more targetplant volatile compounds indicates the presence of a plant diseaseassociated with the one or more volatile compounds. In embodiments, themethod may include using an application that interacts with the signalprocessing mechanism of the detection system and provides informationand/or recommendations based on the information provided by the signalprocessing mechanism based on the data received from the electrochemicalsensor. In embodiments, methods of the present disclosure also includetreating the plant or crop for a disease/pathogen when the volatiledetection system indicates infection by a plant pathogen.

Additional details regarding the methods and compositions of the presentdisclosure are provided in the Examples below. The specific examplesbelow are to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever. Without furtherelaboration, it is believed that one skilled in the art can, based onthe description herein, utilize the present disclosure to its fullestextent. It should be emphasized that the embodiments of the presentdisclosure, particularly, any “preferred” embodiments, are merelypossible examples of the implementations, merely set forth for a clearunderstanding of the principles of the disclosure. Many variations andmodifications may be made to the above-described embodiment(s) of thedisclosure without departing substantially from the spirit andprinciples of the disclosure. All such modifications and variations areintended to be included herein within the scope of this disclosure, andprotected by the following claims.

The following examples are put forth so as to provide those of ordinaryskill in the art with a complete disclosure and description of how toperform the methods and use the compositions and compounds disclosedherein. Efforts have been made to ensure accuracy with respect tonumbers (e.g., amounts, temperature, etc.), but some errors anddeviations should be accounted for. Unless indicated otherwise, partsare parts by weight, temperature is in ° C., and pressure is at or nearatmospheric. Standard temperature and pressure are defined as 20° C. and1 atmosphere.

It should be noted that ratios, concentrations, amounts, and othernumerical data may be expressed herein in a range format. It is to beunderstood that such a range format is used for convenience and brevity,and thus, should be interpreted in a flexible manner to include not onlythe numerical values explicitly recited as the limits of the range, butalso to include all the individual numerical values or sub-rangesencompassed within that range as if each numerical value and sub-rangeis explicitly recited. To illustrate, a concentration range of “about0.1% to about 5%” should be interpreted to include not only theexplicitly recited concentration of about 0.1 wt % to about 5 wt %, butalso include individual concentrations (e.g., 1%, 2%, 3%, and 4%) andthe sub-ranges (e.g., 0.5%, 1.1%, 2.2%, 3.3%, and 4.4%) within theindicated range. In an embodiment, the term “about” can includetraditional rounding according to significant figures of the numericalvalue. In addition, the phrase “about ‘x’ to ‘y’” includes “about ‘x’ toabout ‘y’”.

EXAMPLES

Now having described the embodiments of the present disclosure, ingeneral, the following Examples describe some additional embodiments ofthe present disclosure. While embodiments of present disclosure aredescribed in connection with the following examples and thecorresponding text and figures, there is no intent to limit embodimentsof the present disclosure to this description. On the contrary, theintent is to cover all alternatives, modifications, and equivalentsincluded within the spirit and scope of embodiments of the presentdisclosure

Example 1—Bi-enzyme Sensor Detection of Methyl Salicylate with AO andHRP

The present example and accompanying drawings describe an embodiment ofa electrochemical biosensor of the present disclosure for detectingmethyl salicylate, such as illustrated in FIG. 9. An embodiment of amethyl salicylate electrochemical biosensor can be constructed usingenzyme functionalized nanomaterial composite that selectively detectshydrolyzed methyl salicylate. The conduction of the biosensor isdescribed below.

A screen-printed carbon electrode (strip electrode) was modified withmulti-walled carbon nanotubes. The carbon nanotubes provide the highsurface area for enzyme-electrochemical reactions, while at the same actas immobilization support for the enzymes. Two enzymes, alcohol oxidase(AO) and horseradish peroxidase (HRP) (bio-recognition elements) wereimmobilized onto the carbon nanotubes (transducer), using ahetero-bifunctional tethering agent, namely 1-pyrene butanoic acidsuccinimidyl ester (PBSE) as shown in FIG. 10.

The resulting bi-enzyme carbon nanotube composite carries out a cascadicconversion of hydrolyzed methyl salicylate into hydrogen peroxide. Thiscascadic conversion proceeds as follows: The first step involves thehydrolysis of methyl salicylate into methanol and salicylic acid inbasic medium (NaOH) as illustrated in step (1) of FIG. 9. The secondstep involves the enzymatic oxidation of methanol to formaldehyde andsimultaneous reduction of oxygen to hydrogen peroxide by the enzymealcohol oxidase as illustrated in step (2) of FIG. 9. The third stepinvolves the selective reduction of hydrogen peroxide to water using acoupled electro-enzymatic reaction by the enzyme horseradish peroxidaseas illustrated in step (3) of FIG. 9. The hydrogen peroxide reductionresults in the amperometric signal, which can be detected at theelectrode.

This electrode was attached to a standard 3 electrode cell, withpotentiostat, and changes in amperometric signal at the workingelectrode (volatile detection electrode) were measured by cyclicvoltammetry as shown in the figures. Details about fabrication and theexperimental conditions and results are presented below an in FIGS.9-20.

Experimental

Materials

Alcohol Oxidase (EC 1.1.3.13) solution from Pichia pastoris waspurchased from Sigma-Aldrich and used as it received. HorseradishPeroxidase with activity 281.0U/mg was purchased from Calbiochem.Multiwall carbon nanotube (MWNT) was obtained from DropSens.1-pyrenebutanoic acid, succinimidyl ester (PBSE) was purchased fromAnaSpec Inc., Fremont Calif. Dimethylformamide (DMF) was purchased fromAcros Organics. Chemicals for interference study like cis-3-hexenol,hexyl acetate and cis-hexen-1-yl acetate obtained from TCI America(Portland, Oreg.) were used as received. Wintergreen oil purchased fromPiping Rock Health Products, LLC was used as obtained for real samplestudy. Methyl salicylate (MeSA) was used as received from Sigma-aldrich.All other chemicals were used as analytical grade. 100 mM phosphatebuffer (pH 7.6) was prepared for all experiments. All the aqueoussolutions were prepared using 18.2 MΩ nano pure de-ionized (DI) water.Solutions were oxygenated by purging with purified oxygen for 15 minbefore each experiment.

Apparatus

Cyclic voltammetry and constant potential amperometry was performedusing CHI 920c potentiostat. A conventional three-electrode systemhaving a Pt wire as the counter electrode and 3 M Ag/AgCl as thereference electrode was used for electrochemical measurement. Theworking electrodes, both modified with multiwall carbon nanotube (MWNT),1-Pyrenebutanoic acid, succinimidyl ester (PBSE), alcohol oxidase (AO)and horseradish peroxidase (HRP) are glassy-carbon (GC) electrode fromCH Instrument, Inc. for cyclic voltammetry and rotating disk electrode(RDE) from Pine Instrument Company for constant potential amperometry.Rotating electrode speed control was also used. All experiments werecarried out at 25±2° C.

Electrode preparation and CV, I-t Measurement

Enzyme functionalized nanocomposite electrodes were prepared asillustrated in FIG. 10. GC and RDE electrodes were first polished onpolishing pad with 0.05 micron allumina polishing powder from CHInstrument Inc. before each experiment. The electrodes were thensonicated and rinsed by DI water to remove the fine powder adhered tothe electrode surface. MWNT suspension was prepared by ultrasonicationof 1 mg of MWNT in 1 mL DMF for 1 hour. The MWNT modified electrodeswere prepared by drop casting 8 μL (in 8 steps of 1 μL) for GC electrodeand 12 μL (in 3 steps of 4 μL) for RDE followed by drying at 70° C. MWNTmodified electrodes were placed on the ice and allowed to cool downbefore 2 μL and 4 μL of 10 mM PBSE in DMF were added on GC and RDErespectively. The electrodes were incubated for 15 min to allow thenon-covalent linkage between MWNT and PBSE. Then the electrodes wererinsed by DMF and 100 mM PBS pH 7.6 sequentially to remove excessivePBSE. HRP solution was prepared by weighing 5 mg HRP dissolving into 1mL 20 mM PBS pH 7.6. The bi-enzyme solution was simply prepared bymixing 5 μL of alcohol oxidase solution and 5 μL HRP solution. 10 μL ofbi-enzyme solution was drop casted on the electrodes and incubated onice for 30 min for enzyme immobilization. The electrodes were rinsedwith 100 mM PBS pH 7.6 to remove unimmobilized enzyme.

For cyclic voltammetry, the potential range for bi-enzyme modified GCwas performed from 0.7 V to 0.2 V with scan rate 20 mVs⁻¹ and sampleinterval 0.001 V for one cycle. The initial potential for amperometricl-t curve collected by constant potential amperometry with rotating discelectrode was 0.45 V with 0.1 s interval for data collection.

Hydrolysis of Methyl Salicylate

The methyl salicylate was mixed with 0.1866 M KOH in 15 mL falcon tube.The falcon tube was sealed and placed in boiling water bath for 30minutes hydrolysis. Then the falcon tube was cooled down to roomtemperature before adding phosphoric acid to neutralize the pH to 7.6.

Results and Discussion

Electrochemical Response of Methanol on Bi-Enzyme Modified Electrode

The bi-enzyme modified GC electrode prepared as described above wascharacterized by 3 mM methanol in electrochemical cell by CV. In orderto compare the behavior of bi-enzyme modified GC electrode, monoenzymemodified GC (GC electrodes only modified by alcohol oxidase orhorseradish peroxide) were also characterized by 3 mM methanol,respectively. In the FIG. 11(a), the reduction of hydrogen peroxidecatalyzed by HRP was observed with an onset of reduction of 0.6 V andpeak current of 0.45 V. However, by only modification of AO or HRP, nosignificant redox peak can be observed, this demonstrates methanolcannot be directly detected on the electrode with no functional enzymeto catalyze reaction, nor by hydrogen peroxide produced by AO withoutfurther electron transfer. However, the possibility also exists that theproduced formaldehyde after oxidation of methanol with AO can be reducedon the electrode to display reduction signal. Moreover, the methanolbased methyl salicylate detection can also be interfered by salicylicacid produced after the hydrolysis of methyl salicylate. Thus, theexperiment was also carried out to illustrate that the producedformaldehyde by methanol oxidation by AO does not have anyelectrochemical signal because no redox peak appears at the potentialrange of interest (FIG. 12). The salicylic acid produced afterhydrolysis of methyl salicylate, although detectable through oxidationand reduction, is much weaker and negligible compared to theelectrochemical signal from methanol (FIG. 12). It demonstrates themethanol is the only compound which can be reliably detected on thebi-enzyme modified electrode, and further illustrates that the methanolcan be detected on the electrode and corresponds to the amount of methylsalicylate in the sample.

Electrochemical Response of Hydrolyzed Methyl Salicylate on Bi-EnzymeModified Electrode

Similarly, the behavior of bi-enzyme modified GC electrode prepared asstated in introduction section was evaluated by 1.88 mM hydrolyzedmethyl salicylate in electrochemical cell by CV. In order to evaluatethe function of bi-enzyme modified sensor compared to the monoenzyme andnon-enzyme modified cases, in addition to the bi-enzyme modified GCelectrode, monoenzyme and non-enzyme modified (AO modified only, HRPmodified only, and no enzyme modified GC electrodes) were also evaluatedto determine the behavior of bi-enzyme modified electrode for methylsalicylate measurement. Similar results to methanol evaluation can beobtained from FIG. 11B, which demonstrates the bi-enzyme modifiedelectrode can be applied for methyl salicylate detection based on thereduction peak of hydrogen peroxide appears at 0.45 V. In this reaction,the hydrolyzed methyl salicylate containing methanol was added into theelectrochemical cell. The methanol was oxidized to formaldehyde andoxygen was reduced to hydrogen peroxide, which further obtained theelectron from the electrode and displays the reduction current. Thenon-enzyme modified and monoenzyme modified cases do not have thesimilar electrochemical reduction signal obtained from the bi-enzymemodified electrode. In the AO modified-only case, although methanolwithin the hydrolyzed methyl salicylate can be oxidized and hydrogenperoxide was produced as the product, hydrogen peroxide was not quiteelectrochemically active among that potential region which limits theelectrochemical signal directly produced by hydrogen peroxide reductionwithout the help of HRP. In the HRP modified-only and non-enzymemodified only cases, methanol can neither react with HRP nor directlyreduced on the electrode to give out signal. Additionally, salicylicacid, which is also present in the sample, can give littleelectrochemical signal. This demonstrates only the bi-enzyme modifiedelectrode can be applied to methyl salicylate detection.

Enzyme ratio of AO and HRP was also optimized to achieve the highestsensitivity of methyl salicylate detection. This was carried out bydifferent combination of enzyme ratios (2.8 U AO/14.0 U HRP, 5.5 UAO/7.0 U HRP and 11.0 U AO/3.5 U HRP). The results showed 5.5 U AO/7.0 UHRP (5 μL AO and 5 μL 5 mg/mL HRP) was the ratio providing highestsensitivity and will be applied to modify the electrode for followingquantitative determination FIG. 13).

Electrochemical Responses of Hydrolyzed Methyl Salicylate by CyclicVoltammetry and Constant Potential Amperometry

The GC electrode was prepared and modified as described in experiment.Hydrolyzed methyl salicylate was added stepwise to form differentconcentration gradient from 1 μM to 3 mM followed by the cyclicvoltammetry measurement. The above concentration range was chosen by aseries of experiments, where the lowest limit was determined based onthe noticeable increase in reduction current upon an incrementaladdition of hydrolyzed methyl salicylate into the electrolyte.Similarly, the upper concentration limit was chosen based on the rate ofdecrease in oxidation current during subsequent additions of hydrolyzedmethyl salicylate. FIG. 14(a) indicates the CV without addition ofhydrolyzed methyl salicylate does not display any redox peak due to noreactant in the electrochemical cell. As the concentration of hydrolyzedmethyl salicylate increased to 10 μM, small reduction peak for hydrogenperoxide was observed at 0.6 V and the peak current (I_(pc)) increasesand peak potential (E_(pc)) moves negatively to 0.45 V due to theincreasing concentration of hydrolyzed methyl salicylate in theelectrochemical cell. On the other hand, an oxidation peak around 0.5 Vcan be observed when the concentration of hydrolyzed methyl salicylatereaches 1 mM and the oxidation peak increases as the concentration ofhydrolyzed methyl salicylate increased to 3 mM. This could be attributedto the oxidation of salicylic acid after hydrolysis of methylsalicylate. The electrochemical data such as sensitivity, limit ofdetection (LOD), limit of quantification (LOQ), were derived fromequation 1, 2 and 3 (SD is standard deviation), as well as linear rangeand initial response (Table 2).

$\begin{matrix}{\mspace{79mu} {{Sensitivity} = \frac{{Slope}\mspace{14mu} {of}\mspace{14mu} {calibration}\mspace{14mu} {curve}\mspace{14mu} \left( {A\; M^{- 1}} \right)}{{Area}\mspace{14mu} {of}\mspace{14mu} {electrode}\mspace{14mu} \left( {cm}^{2} \right)}}} & (1) \\{{LOD} = {3.3 \times \frac{{SD}\mspace{14mu} {of}\mspace{14mu} {peak}\mspace{14mu} {current}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {absence}\mspace{14mu} {of}\mspace{14mu} {analyte}\mspace{14mu} (A)}{{Slope}\mspace{14mu} {of}\mspace{14mu} {linear}\mspace{14mu} {calibration}\mspace{14mu} {curve}\mspace{14mu} \left( {A\; M^{- 1}} \right)}}} & (2) \\{{LOQ} = {10 \times \frac{{SD}\mspace{14mu} {of}\mspace{14mu} {peak}\mspace{14mu} {current}\mspace{14mu} {in}\mspace{14mu} {the}\mspace{14mu} {absence}\mspace{14mu} {of}\mspace{14mu} {analyte}\mspace{14mu} (A)}{{Slope}\mspace{14mu} {of}\mspace{14mu} {linear}\mspace{14mu} {calibration}\mspace{14mu} {curve}\mspace{14mu} \left( {A\; M^{- 1}} \right)}}} & (3)\end{matrix}$

Although CV provides firsthand information of electrochemistry,biosensor applications demand chronoamperometric to eliminate the noisecaused by the capacitance and resistance in order to improve overallelectroanalytical measurement accuracy. Therefore, constant potentialamperometry was performed with RDE. The modified RDE prepared asintroduced in experiment was stabilized until 300 second and hydrolyzedmethyl salicylate was added stepwise at 50 second interval to generatedifferent concentration gradient from 0.1 μM to 1 mM. The methodology ofdetermining the concentration range was similar to that for CV above.The first observable stepwise increase appears at the concentration of0.5 μM as it shows in FIG. 14(b). The comparison of electrochemical datawas also conducted (Table 2).

The data indicates higher sensitivity for constant potential amperometry(282.82 μA cm⁻² mM⁻) as opposed to that for cyclic voltammetry (112.37μA cm⁻mM⁻¹). Furthermore, the limit of detection for constant potentialamperometry can be lowered down to 0.98 μM as opposed to cyclicvoltammetry (22.95 μM) which uses about 1.05 hours accumulating enoughsample for detection in a 2 mL electrochemical cell, given that themethyl salicylate production rate is 283 ng hr⁻¹ plant⁻¹ (Table 2).Though the linear range for constant potential amperometry is narrowerthan cyclic voltammetry, it is more favored for sensor development dueto its high sensitivity and low limit of detection.

Interference of Other Green Leaf Volatiles in Methyl SalicylateDetermination

Methyl salicylate is not the only plant volatile that will be producedduring the plant infection. The infected plant volatiles contain othercompounds which are non-specific to the infection are often released atmuch higher concentration. A representative set ofcompounds—cis-3-hexenol, hexyl acetate and cis-3-hexenyl acetate werestudied by constant potential amperometry because they are common tomost plants. The fungus infection usually results in production of 10 μMof cis-3-hexenol, 1.2 μM of hexyl acetate and 20 μM of cis-3-hexenylacetate.²³ Constant potential amperometry with RDE was performed toimplement interference study. The RDE was preconditioned until 300second and hydrolyzed methyl salicylate was added to maintain 50 μM ofworking concentration which is at the middle of linear range. The RDEwas further stabilized to 500 second and hydrolyzed green leaf volatileswere added to maintain the working concentration from 10 μM, 50 μM to 1mM at 50 second interval. The experiment was conducted for all threegreen leaf volatiles respectively and by adding the same volume of 100mM phosphate buffer (pH 7.6) as the control. In the interference study,instead of using the common concentration of green leaf volatiles,stepwise addition of green leaf volatiles until 1 mM was conducted.

From FIG. 15, the absolute reduction current density decreases as morephosphate buffer was added into electrochemical cell due to the dilutionof hydrolyzed methyl salicylate. However, after adding hexyl acetate,although current density also decreases, it does not decrease as thesame pace of control. After adding cis-3-hexenol and cis-3-hexenylacetate, instead of decreasing, current density increased, whichindicates the presence of interference from the three green leafvolatiles. Data further indicate the current density interferenceincrease from 0 (when concentration of green leaf volatile is 0) to10.30%, 6.50% and 11.59%, respectively (when concentration ofcis-3-hexenol, hexyl acetate and cis-3-hexenyl acetate increases to 1mM) (Table 3). Although there appeared to be high interference, 1 mM ismuch higher than the maximum green leaf volatile production in naturalcases. In presence of 100 μM green leaf volatile, which is alsoreasonably higher than the usual production of 10 μM of cis-3-hexenol,1.2 μM of hexyl acetate, and 20 μM of cis-3-hexenyl acetate, results inonly 2.36%, 2.14% and 3.43%. With all the interference less than 5.0% atthe interference concentration of 100 μM, it is obvious that theinterference from green leaf volatiles is not significant in theexpected natural situation, although slight interference can be observedin the experimental conditions. The interference from cis-3-hexenol maybe attributed to the activity of alcohol oxidase to cis-3-hexenol,although the activity towards cis-3-hexenol is weaker compared tomethanol due to its long carbon chain. The same phenomenon was observedfrom hexyl acetate, whose hydrolysis product also containscis-3-hexenol. The interference from cis-3-hexenyl acetate might resultfrom the electrochemical activity of cis-3-hexene after the hydrolysis.

Stability and Repeatability

A qualified electrode should also have satisfactory stability. Constantpotential amperometry was conducted with 50 μM hydrolyzed methylsalicylate on Day 0, and the net current density was calculated from thecurrent density before and after addition of the hydrolyzed methylsalicylate. The RDE was stored in 100 mM phosphate buffer (pH 7.6) at 4°C. The experiments were repeated on Day 1, 2, 4, 7 and 10 by using thesame method. The days were decided by each measurement until asignificant decrease of reduction was observed. FIG. 16 shows thecurrent density decreased gradually from Day 0 to Day 10. In Day 1, thecurrent dropped from 100% to 95.27%. However, from Day 1 to Day 4, theelectrode behaved relatively stable with retained current density rangesfrom 94.27% to 98.52%, which could be attributed to the measurementvariances rather than losing the stability itself. The study furtherindicates the electrode was relatively stable for 7 days after thebi-enzyme modified sensor was fabricated, given that more than 90% ofthe current density was retained. However, on Day 10, the retainedcurrent density sharply decreased to 79.13% which might be attributed tothe loss of enzyme activity (Table 4). Although the bi-enzyme basedsensor stability decreased after 7 days, it had more than 90% activityduring the first week after the enzymatic sensor was fabricated. Thisstability is considered satisfactory, particularly if the sensor isimplemented as a one-time use strip.

The repeatability test was also performed to evaluate the datareliability for each electrode prepared. Bi-enzyme modified RDE was usedto carry out the measurement with 50 μM hydrolyzed methyl salicylate asillustrated in FIG. 17. Although similar current density was observedinitially, after adding 50 μM hydrolyzed methyl salicylate, the currentdensity behaves differently, which could be attributed to differenceamount or orientation in enzyme that attached on the electrode andslight difference of each experiment condition. Difference before andafter adding hydrolyzed methyl salicylate was used for relative standarddeviation (RSD) calculation. The results displays the RSD obtained as6.6% (Table 5), which is acceptable based on the enzyme modifiedelectrode due to the random orientation and slight differences in eachelectrode preparation.

Real Sample Study

Real sample study was used for evaluating the behavior of the electrodesof the present example for field application. Wintergreen oil, which isextracted from wintergreen and essentially contains 98% of methylsalicylate, was hydrolyzed as stated before. However, methyl salicylate,which is the main compound in wintergreen oil, is not present in theplant initially. It is only produced enzymatically from a glucosidewithin the leaves when they are macerated in warm water. In thisexperiment, wintergreen oil was introduced to simulate the situationwhen plant produces methyl salicylate due to plant infection. Differentamounts of hydrolyzed wintergreen oil with known concentration of MeSA(standard concentration provided in Table 1) were added into theelectrochemical cell, and current was collected and compared standardI-t curved collected from pure MeSA (black curve) to calculate MeSAconcentration (Table 1). Recovery was determined from calculatedconcentration divided by standard concentration.

Three replicates were repeated by adding hydrolyzed wintergreen oil forI-t curve collection. The trend is almost the same as pure methylsalicylate (FIG. 18). From Table 1, RSD ranges from 91% to 136%, whichdemonstrates the capability of the bi-enzyme modified electrode formethyl salicylate detection in natural case. The bias might be derivedfrom other unexpected electrochemical active compounds in wintergreenoil such as α-pinene, myrcene, δ-3-carene, limonene, 3,7-guaiadiene andδ-cadinene reacted directly on electrode. From the RSD in Table 1, itwas demonstrated that the real sample experiments were highly repeatablewhen the concentration is at the center of the linear range due to lessthan 5%. However, the repeatability decreased when the concentrationpushes to the edge of the linear range (upper and lower linear range).

Conclusion

An embodiment of a bi-enzyme modified electrode as a biosensor formethyl salicylate was successfully fabricated and characterized withhydrolyzed methyl salicylate by cyclic voltammetry and constantpotential amperometry. Constant potential amperometry demonstrateshigher sensitivity and lower limit of detection, which allows short-timesample collection for methyl salicylate. The interference study withthree green leaf volatile indicates the biosensor for methyl salicylatedetection will not be significantly interfered by green leaf volatile.The biosensor also displays satisfactory stability and acceptablerepeatability. Wintergreen oil which was used for real sample studyfurther proved the biosensor fabricated in this example was suitable foron-field demonstration.

TABLE 1 Standard concentration, calculated concentration, recovery, SDand RSD of hydrolyzed Wintergreen oil for real sample study Std. Con.(μM) Cal. Con. (μM) Recovery (%) SD (μM) RSD (%) 2.45 2.24 91 0.28 12.384.90 5.78 118 0.28 4.90 9.80 12.64 129 0.29 2.29 24.5 31.38 128 0.963.06 49.0 59.81 122 2.71 4.53 98.0 133.09 136 25.36 19.05

TABLE 2 Electrochemical data for hydrolyzed methyl salicylate from CVand l-t curve Sensitivity Linear Initial (μA cm⁻² LOD LOQ Range ResponseElectrode mM⁻¹) (μM) (μM) (mM) (μM) CV GC 112.37 22.95 69.55 0-1.0 10.0l-t Curve RDE 282.82 0.98 2.97 0-0.1 0.5

TABLE 3 Current density interference of different concentration of greenleaf volatiles in 50 μM in hydrolyzed methyl salicylate Current densityinterference percentage in 50 μM hydrolyzed methyl salicylate (%)Concentration cis-3- of GLV (μM) cis-3-Hexenol Hexyl Acetate HexenylAcetate 0 0 0 0 10 0.03 0.39 0.37 50 0.98 0.22 −0.03 100 2.36 2.14 3.43250 3.04 1.94 3.46 500 6.11 3.49 6.95 1000 10.30 6.50 11.59

TABLE 4 Stability study of bi-enzyme modified electrode with 50 μMhydrolyzed methyl salicylate Current Density Current Density Time (Day)(μA cm⁻²) Retained (%) 0 16.42 100 1 15.64 95.27 2 16.17 98.52 4 15.4794.27 7 14.85 90.44 10 12.99 79.13

TABLE 5 Repeatability study of bi-enzyme modified electrode with 50 μMhydrolyzed methyl salicylate Background Current MeSA Current Net CurrentDensity Density Density Experiment (μA cm⁻²) (μA cm⁻²) (μA cm⁻²) #1−0.18 −18.31 18.13 #2 0.08 −15.77 15.85 #3 −0.95 −19.16 18.21 #4 −0.78−16.77 15.99 #5 −0.21 −18.79 18.59 #6 −0.41 −16.79 16.39 Average 17.19SD 1.14 RSD 6.6%

Example 2—Electrochemical Detection of p-ethylguaiacol on Metal OxideNanoparticle Sensor

This example describes an embodiment of biosensor electrodesfunctionalized with metal oxide nanoparticles for detection ofp-ethylguaiacol. In other embodiments of the present electrodesdescribed in this example, the metal oxide nanoparticles can also befunctionalized with enzymes specific for target stress-induced plantvolatiles and/or target plant pathogen-emitted volatile compounds.

The use of metal oxide nanoparticles as the nanoparticle transducermaterial for an electrode detection surface was explored due to theiradvantages compared to commonly used nanomaterials. The advantagesinclude the following: metal oxide nanoparticles are catalyst for thedehydrogenation of alcoholic compounds such as aliphatic alcohols,acetic acid etc., which could enhance the plant volatile reaction on thetransducer; metal oxides are inexpensive compared to noble metalnanoparticles; some metal oxides have large band gap (greater than 3.3eV), which can enable them for amperometric signal generation in aqueoussolution; and the preparation methods for obtaining desired sizes andshapes of these nanoparticles are easier compared to other nanostructuresynthesis. This example describes embodiments with one of two metaloxides, SnO₂ or TiO₂, as electrochemical detection elements foramperometric sensing. Screen-printed carbon (SP) electrodes weremodified with nanoparticles of SnO₂ or TiO₂ and used for theelectrochemical detection of p-ethylguaiacol in simulated fruit volatilesamples.

Experimental

Materials

Tin (IV) oxide (<100 nm) and titanium (IV) oxide (˜21 nm) nanoparticlesobtained from Sigma-Aldrich were used for preparing nanoparticlesuspensions. p-ethylguaiacol from Frinton Laboratories, Inc., (NewJersey, US) was used as received. p-ethylphenol was purchased fromSigma-Aldrich and used for interference and synthetic real samplestudies. cis-3-hexenol, hexyl acetate, cis-hexen-1-yl acetate,1-octen-3-ol and 3-octanone obtained from TCI America (Portland, Oreg.)were used as received. All other chemicals used were of analyticalgrade. All aqueous solutions were prepared using 18.2 MΩ nanopurede-ionized (DI) water. 0.1M electrolyte solution of potassium hydrogenphthalate (KHP) (pH 4) was prepared for all the experiments. Solutionswere deoxygenated by purging with pre-purified nitrogen gas for 15 minbefore each electrochemical measurement.

Apparatus

The working electrode was a screen-printed carbon electrode (SP)modified either with SnO₂ nanoparticles or TiO₂ nanoparticles. All otherconditions were the same as described in Example 1, above.

Electrode Preparation

SnO₂ and TiO₂ nanoparticle suspensions were prepared by ultrasonicationof 1 mg of the respective nanoparticles in 1 mL DI water. The SnO₂ andTiO₂ nanoparticle modified SP electrodes were prepared by drop casting18 μL (in three steps of 6 μL additions) of the nanoparticle suspensionon the SP, followed by drying at 70° C. The cyclic voltammogram (CV) anddifferential pulse voltammogram (DPV) studies were done in a 10 mLvoltammetry cell containing N₂ saturated 0.1M potassium hydrogenphthalate (KHP) electrolyte for SnO₂ as well as TiO₂. The potentialrange scanned for CV studies were for metal oxide modified SP electrodesat the scan rate of 0.02 Vs⁻¹. For DPV the potential scanned was from−0.1 to 0.7 V with the increment of 0.004 V, amplitude 0.05 V, pulsewidth 0.2 s and pulse period 0.5 s for all electrodes.

Results and Discussion

Electrochemical Response of p-ethylguaiacol on Metal Oxide Modified SPs

The metal oxide modified electrodes were characterized using cyclicvoltammetry in the presence and absence of p-ethylguaiacol. Althoughacidic conditions favor p-ethylguaiacol oxidation as found in ourexperiments (data not shown), a pH 4 KHP electrolyte was used in ourstudies to avoid reaction between metal oxides and electrolyte. Thecyclic voltammograms of SnO₂ and TiO₂ modified electrodes in thepresence and absence of p-ethylguaiacol are shown in FIG. 19A and theresults demonstrate the better sensitivity of p-ethylguaiacol detectionby metal oxide nanoparticle modified electrodes when compared withunmodified screen printed (SP) carbon electrode. In the absence ofp-ethylguaiacol, TiO₂ showed no redox activity, and SnO₂ exhibited broadredox peaks in the potential window of −0.1 to 0.4 V, which correspondto the adsorption and desorption of phthalate ions, a known behavior forSnO₂ in KHP electrolyte. In the presence of p-ethylguaiacol, both metaloxides exhibited irreversible peaks at 0.62 V (oxidation) and at 0.2 V(reduction). The irreversible oxidation of p-ethylguaiacol at 0.62V,occurs as per equation 1, where p-ethylguaiacol forms phenoxy radicalintermediate, which then reacts with phthalate anion in the electrolyteto form benzoic acid derivative and H₃O⁺ (second step in equation 4).The irreversible reduction peak in the cyclic voltammograms at 0.2 Vcould be due to the reduction of phenoxy radical to p-ethylguaiacol. Theresult suggests that at potentials below 0.2 V, the p-ethylguaiacoloxidation is reversible.

Comparison of p-ethylguaiacol oxidation peaks (at 0.62 V) shows bothSnO₂ and TiO₂ possess similar oxidation currents. The effect ofp-ethylguaiacol concentration on the oxidation currents was studied andreported in FIGS. 19B and 19C for SnO₂ and TiO₂, respectively. Thestepwise increase in p-ethylguaiacol concentration from 0.2 μM to 2.6 mMin the electrochemical cell was achieved by adding p-ethylguaiacol fromthe series of standard concentrations. The above concentration range waschosen by a series of experiments, where the lowest limit was determinedbased on the noticeable increase in oxidation current upon anincremental addition of p-ethylguaiacol into the electrolyte. Similarly,the upper concentration limit was chosen based on the rate of decreasein oxidation current during subsequent additions of p-ethylguaiacol. Thecyclic voltammetry results in FIGS. 19B and 19C show that increasingconcentration of p-ethylguaiacol increased the oxidation peak current(I_(pa)) of p-ethylguaiacol oxidation on both SnO₂ and TiO₂ electrodes.The initial response to p-ethylguaiacol additions showed a shift in thep-ethylguaiacol oxidation peak potential (E_(pa)) from 0.62 V to 0.7 V.This could be attributed to the increase in acidity of the electrolytedue to more H₃O⁺ formation (equation 4).

The electroanalytical data such as sensitivity, limit of detection (LOD)at the signal to noise ratio of 3, and limit of quantification (LOQ) ofboth SnO₂ and TiO₂ electrodes derived using the equations 1, 2, and 3from Example 1, above, and are listed in Table 6 respectively.

Comparison of sensitivity values obtained through cyclic voltammogramgiven in Table 6 reveals that SnO₂ had higher sensitivity, lower LOD andLOQ than TiO₂ although the difference was not significant. CV provides afirsthand understanding of the electrochemistry of the system. Biosensorapplications can also use chronoamperometric or pulse methods toeliminate the noise caused by the capacitance and resistance in order toimprove overall electroanalytical measurement accuracy.

Therefore, differential pulse voltammetry (DPV) was used in a similarmanner to CV to study p-ethylguaiacol oxidation between −0.1 and 0.7 V.Compared to unmodified SP electrode, TiO₂ and SnO₂ modified electrodesdisplay higher sensitivity for p-ethylguaiacol detection (data notshown). Similar to CV results, DPV also showed peaks in the absence ofp-ethylguaiacol on SnO₂, due to the adsorption and desorption ofphthalate ions. In the presence of p-ethylguaiacol, the oxidation peaksappeared at 0.54 V (E_(pa)) with similar I_(pa) values for both theelectrodes as shown in FIG. 20A. The p-ethylguaiacol characteristicpeaks for both SnO₂ and TiO₂ were similar to that of cyclicvoltammograms with ˜0.05 V negative shift due to the applied amplitude(0.05 V) during DPV measurements. The peaks currents (I_(pa)) forp-ethylguaiacol oxidation increased with the concentration in the rangeof 0.2 μM to 1.5 mM on both electrodes as shown in FIGS. 20B and 20C.The inset figures show a linear dependency of I_(pa) on concentration.The empirical electroanalytical values derived from the DPV data arealso given in Table 6. Due to the elimination of capacitance as well asadsorption-desorption effects in DPV, the values for DPV showed lowersensitivity, but better detection and quantification limits for bothelectrodes when compared to their corresponding CV values. However TiO₂exhibited better sensitivity and detection limits than SnO₂ according tothe DPV results, although the difference is not significant (Table 6).DPV values are better representative of the sensing characteristic ofthe electrodes due to the elimination of parasitic currents from thetrue oxidation response of p-ethylguaiacol. The results suggest thatboth SnO₂ and TiO₂ could be used to construct amperometric sensors forp-ethylguaiacol detection at concentrations relevant to typical infectedfruit volatiles.

Reproducibility and Re-Usability Studies

Eight SnO₂ and TiO₂ modified SP electrodes were prepared under similarconditions and tested for p-ethylguaiacol oxidation using DPV. The DPVpeak currents (I_(pa)) at 0.54 V, for all eight electrodes were measuredat concentration of 2.5 mM. The high concentration was chosen to ensurethat even subtle changes in measured currents can be determined. Theresults (data not shown) showed that the peak current for all eightelectrodes varied within 2.48 and 4.85% for SnO₂ and TiO₂ respectively.The low variability indicates high reproducibility of the observedresults for both electrodes.

The reusability or stability of SnO₂ and TiO₂ modified SP electrodeswere tested in a series of DPV experiments at 2.5 mM p-ethylguaiacolconcentration on consecutive days for a period of 15 days. I_(pa) ofp-ethylguaiacol oxidation in DPVs was measured on each day andpercentage decrease in current decrease over time was calculated fromthe measurements (data not shown). The results showed a loss of activityup to 67% for SnO₂ and 81% for TiO₂ after 15 days. Though the currentsdecreased significantly over time, the rate of decrease slowed downafter the first two days with no large decrease beyond the first week.The loss in stability could be attributed to the formation of surfaceoxides and other adsorption effects from the ions present in theelectrolyte that tend to slowly poison the electrode over the long-term.

Interference of Other Plant Volatiles in p-ethylguaiacol Determination

The infected plant volatile contains other chemical compounds that arenon-specific to the infection and often released at much higherconcentrations than p-ethylguaiacol. A representative set of suchcompounds was selected and their interference effects on p-ethylguaiacolmeasurements were studied using DPV. The compounds used to studyinterference were p-ethylphenol, 3-octanone, 1-octen-3-ol,cis-3-hexenol, hexyl acetate and cis-hexen-1-yl acetate. p-ethylphenol,3-octanone and 1-octen-3-ol are present in the chemical signature of thePhytophthora cactorum itself. The other three compounds cis-3-hexenol,hexyl acetate and cis-hexen-1-yl acetate are green leaf volatiles thatare common to most plants. The fungi infected plant typically release0.2 μM of 3-octanone, 0.2 μM of 1-octen-3-ol, 10 μM of cis-3-hexenol,1.2 μM of hexyl acetate and 20 μM of cis-hexen-1-yl acetate. Thereforethese concentrations were used in our interference study. Theexperiments were conducted separately for each of the six interferingcompounds where the p-ethylguaiacol concentration was kept constant andas low as possible (20.8 μM), but within the linear response (I_(pa))region obtained in DPV. The results showed characteristic peaks forp-ethylguaiacol even in the presence of interfering compounds as shownin FIGS. 21A and 21B for SnO₂ and TiO₂ respectively. On both SnO₂ andTiO₂ electrodes, the addition of p-ethylphenol significantly changed theDPV wave above 0.55 V but not at the peak oxidation potential (0.54 V)of p-ethylguaiacol (FIGS. 21A and 21B). As shown by the calculatedI_(pa) values in Table 7, p-ethylphenol interference was limited to±6.7% for SnO₂ and TiO₂ respectively. Addition of cis-hexen-1-yl acetateshowed less than 2% interference on p-ethylguaiacol signal on TiO₂, butup to 12% interference on SnO₂. The reason for this difference is notclearly understood. Other compounds such as 3-octanone or 1-octen-3-olor cis-3-hexenol or hexyl acetate did not show any significantinterference on the p-ethylguaiacol signal and the interference waslimited to less than 2%. The studies above indicate p-ethylguaiacoldetection on metal oxide modified electrodes does not suffer anysignificant interference from both fungal and green leaf volatilecompounds.

p-ethylguaiacol Determination in Simulated Fruit Volatile

The ability of SnO₂ or TiO₂ for the determination of p-ethylguaiacol inreal infected samples was evaluated using simulated chemical mixturethat mimics the composition of the real fruit volatile signature. Asdiscussed in the previous section, chemical signature from infectedplants will contain both the green leaf volatiles and the volatiles fromthe pathogen itself. Two sets of samples were used for simulation: (i)only infected fruit volatiles and (ii) both infected fruit and greenleaf plant volatiles. The composition of (i) was 20.8 mMp-ethylguaiacol, 2.5 mM p-ethylphenol, 2.5 μ.M 3-octanone and 2.5 μM1-octen-3-ol. The composition of (ii) includes all (i) in addition to 10μM cis-3-hexen-1-ol, 1.25 μM hexyl acetate and 25 μM cis-hexen-1-ylacetate. The above concentrations were chosen based on the compositionof typical chemical signature of Phytophthora cactorum infection. Theexperiments were done using DPV and the p-ethylguaiacol oxidationcurrent was measured for detailed analysis. Parameters such as theconcentrations added in the experiment, found and relative standarddeviation (RSD) obtained from the experiments were calculated from theDPV measurements and are listed in Table 8. The values show that therecovery of p-ethylguaiacol in both simulated samples varied from 91 to101% for both electrodes with RSD values between 4 and 5%. The analysisshows that both SnO₂ and TiO₂ electrodes can be used for p-ethylguaiacoldetermination.

Conclusions

Both SnO₂ and TiO₂ have been demonstrated to show similar detectioncapabilities for p-ethylguaiacol based on amperometric determination.Ultra low limits of detection were achieved by both metal oxideelectrodes in DPV measurement. Both electrodes exhibited goodreproducibility towards p-ethylguaiacol determination. The CV and DPVdata along with the chemical reactions established elucidate theelectrochemical reaction mechanisms pertaining to the amperometricsensing of p-ethylguaiacol. The electroanalytical data provided in thisexample can be used for both qualitative and quantitative determinationof p-ethylguaiacol. The synthetic sample studies presented illustratethe approach for p-ethylguaiacol sensing during initial stages ofPhytophthora cactorum infection.

TABLE 6 Comparison of sensitivity, linear range, LOD and LOQ ofp-ethylguaiacol at different electrodes obtained using differentelectrochemical techniques Linear Sensitivity EPa range (μA cm⁻²Electrode pH Technique (V) (R²) mM⁻¹) LOD (nM) LOQ (nM) SnO₂-SP 4 CV0.62 0.6 μM-0.17 mM 232 82 249 (0.9954) DPV 0.54 0.2 μM-0.1 mM 174 62188 (0.9932) TiO₂-SP 4 CV 0.62 0.6 μM-0.17 mM 200 126 382 (0.9972) DPV0.54 0.2 μM-0.1 mM 188 35 106 (0.9934)

TABLE 7 Interference study of 20.8 μM p-ethylguaiacol with 6 differentcompounds p- ethylphenol, cis-3-hexen-1-ol, hexyl acetate andcis-3-hexen-1-yl acetate, 3-octanone and 1-octen-3-ol by DPV CurrentActivity Electrode Compound Concentration (uA) (%) SnO₂-SP p-ethylphenol0 0.3212 100 2.50 mM 0.3533 110.01 cis-3-hexen-1-ol 0 0.2906 100 32 μM0.2956 101.73 hexyl acetate 0 0.3249 100 2 μM 0.3274 100.76cis-3-hexen-1-yl 0 0.2672 100 acetate 32 μM 0.2972 111.21 3-octanone 00.3301 100 2 μM 0.3320 100.57 1-octen-3-ol 0 0.3381 100 2 μM 0.3436101.62 TiO₂-SP p-ethylphenol 0 0.3459 100 2.50 mM 0.3227 93.3cis-3-hexen-1-ol 0 0.2783 100 32 μM 0.2782 99.96 hexyl acetate 0 0.3060100 2 μM 0.3092 101.08 cis-3-hexen-1-yl 0 0.3336 100 acetate 32 μM0.3400 101.91 3-octanone 0 0.3334 100 2 μM 0.3391 101.70 1-octen-3-ol 00.3278 100 2 μM 0.3308 100.90

TABLE 8 Simulated sample study using typical chemicals released duringPhytophthora cactorum infection of plants Added Found Recovery RSDElectrode Sample (μA) (μA) (%) (%) SnO₂-SP Infected fruit 0.0455 0.041791.65 3.65 0.1942 0.1947 100.26 0.4816 0.4789 99.44 1.5130 1.5110 99.87Infected fruit 0.0455 0.0495 108.79 3.88 with plant 0.1942 0.2011 103.550.4816 0.4816 100.00 1.5130 1.4890 98.41 TiO₂-SP Infected fruit 0.04210.0389 92.40 4.85 0.2218 0.2019 91.03 0.5017 0.5021 100.08 1.6210 1.6500101.79 Infected fruit 0.0421 0.0399 94.77 3.67 with plant 0.2218 0.207093.33 0.5017 0.5067 101.00 1.6210 1.6420 101.30

Example 3—Detection of Methyl Salicylate on Bi-Enzyme ElectrochemicalSensor

This example describes an embodiment of a bi-enzyme functionalizedelectrochemical biosensor of the present disclosure with immobilizedsalicylate hydroxylase and tyrosinase for detection of methylsalicylate.

Example 1, above, described the application of alcohol oxidase (AO) andhorseradish peroxidase (HRP) based bi-enzyme biosensing platform forMeSA detection. This example provides another sensitive and selectiveenzyme combination for bi-enzyme biosensor based on salicylatehydroxylase and tyrosinase, which allows improved sensitivity andprevents unwanted cross-reactions that could result in false positivesignal.

Salicylate, a main compound formed after hydrolysis of MeSA, can beelectrochemically detected using salicylate hydroxylase (SH) as therecognition element with high selectivity. The enzyme is immobilizedthrough a tethering chemistry described that binds the enzyme to themulti-walled carbon nanotubes on the surface of glassy-carbonelectrodes. Although salicylate acts as the natural substrate for SH,other pseudo-substrates such as benzoate derivatives can also becatalyzed by SH. This issue is addressed by employing a secondenzyme—tyrosinase (TYR) as a part of the recognition element, in orderto build an enzyme cascade that provides highly selective MeSA detectionon the electrode. The reaction scheme of the enzyme cascade and themechanism behind electrochemical detection are illustrated as steps 1 to4 in FIG. 22. Salicylate produced from the hydrolysis of methylsalicylate (step 1 in FIG. 22) reacts with SH and generate catechol asthe intermediate (step 2). Catechol can be further oxidizedenzymatically by TYR to produce 1,2-benzoquinone (step 3). Thebiochemically generated 1,2-benzoquinone can then be electrochemicallyreduced to catechol by the electrode thereby regenerating catechol (step4). Therefore, the amperometric detection of salicylate will be realizedthrough measuring the reduction current of 1,2-benzoquinone.

Experimental

Materials

Tyrosinase (E.C. 1.14.18.1) derived from mushroom (lyophilized powder,1000 unit/mg solid), methyl salicylate, and farnescene were purchasedfrom Sigma-Aldrich and used as received. Humulene and trimethylbenzenewere obtained from Aldrich for the experiments. Multiwalled carbonnanotubes (MWCNTs) were obtained from DropSens Inc. 1-pyrenebutanoicacid succinimidyl ester (PBSE) was purchased from AnaSpec Inc. (FremontCalif.). Dimethylformamide (DMF), salicylate and NADH were purchasedfrom Acros Organics. FAD and dichlorobenzene were purchased from AlfaAesar and Eastman respectively. Methanol and phosphoric acid wereobtained from Fisher Scientific. All reagents used in this project wereanalytical grade. 0.1 M phosphate buffer (PB) (pH 7.6) was used as theelectrolyte for all experiments. All the aqueous solutions were preparedusing 18.2 MΩ nano pure de-ionized (DI) water. Solutions were oxygenatedby purging with purified oxygen for 15 min before each experiment.

Recombinant Synthesis of Salicylate Hydroxylase

Salicylate hydroxylase enzyme is not commercially available andtherefore was synthesized recombinantly in this work. Gene nahG thatcodes salicylate hydroxylase in Pseudomonas putida can be found fromprevious publications (You et al. Nucleotide sequence analysis of thePseudomonas putida PpG7 salicylate hydroxylase gene (nahG) and its3′-flanking region. Biochemistry 30(6), 1635-1641, which is herebyincorporated by reference herein). The nahG gene was codon optimized forexpression in E. coli and synthesized by GenScript with histidine tag(6X) at N- terminal of the sequence. The recombinant plasmidpTrc99A-nahG was constructed by cloning the nahG gene into pTrc99A thatharbors ampicillin resistance gene (ampR) as an antibiotic selectionmarker. The expression of nahG gene was under the control of P_(lac) andwas inducible by addition of isopropyl β-D-1-thiogalactopyranoside(IPTG). Recombinant plasmid was transformed to E. coli XL1-blue throughelectroporation for the purpose of enzyme expression. The resultanttransformants of E. coli XL1-blue was cultured in test tubes, where eachcontains 3 mL of LB media (with 100 μg/mL of ampicillin). The strain wascultured overnight aerobically at 37° C. Each of the overnight culturewas further inoculated into 250 mL fresh LB media (with 100 μg/mL ofampicillin) and left to grow at 37° C. until OD₆₀₀ reached 0.6. 1 mM ofIPTG was added to initiate the expression of salicylate hydroxylase at20° C. for 8 hours. Cell pellets were collected by centrifugation. Thencell pellets were rinsed with 20 mM PB pH 7.6 twice to remove the LBmedia before being lysed through French Press. The supernatant wascollected as crude extract by centrifugation. The crude extract waspurified through fast-protein liquid chromatography with HisTrapTM HPcolumn. Different segments of eluent were tested by a traditional enzymeassay with addition of FAD and cofactor (White-Stevens and Kamin, 1972,Studies of a flavoprotein, salicylate hydroxylase I. Preparation,properties, and the uncoupling of oxygen reduction from hydroxylation.Journal of Biological Chemistry 247(8), 2358-2370 ; Yamamoto et al.,1965, Salicylate hydroxylase, a monooxygenase requiring flavin adeninedinucleotide I. Purification and general properties. Journal ofBiological Chemistry 240(8), 3408-3413, both of which are incorporatedby reference herein). The segment with highest enzyme activity was addedwith glycerol to final concentration of 20%. The enzyme stock was frozenand stored at −80° C. for all the experiments.

Apparatus

Cyclic voltammetry (CV) and constant potential amperometry (CPA) wereperformed using CHI 920c model potentiostat. Three-electrode system,including a 3 M Ag/AgCI reference electrode, a platinum wire counterelectrode, and a glassy carbon (GC) working electrode purchased fromPine Instrument Company were used for electrochemical measurements in aconventional glass voltammetry cell. All experiments were conducted at22±2° C.

Electrode Preparation and Electrochemical Measurement

GC was first polished on polishing pad with 0.05 micron aluminapolishing powder before each experiment. The electrode was cleaned inthe ultrasonic cleaner for 5 minutes to remove the polishing powderadhered to the surface of the electrode. The electrode was rinsed withDI water before surface modification with carbon nanotubes (CNTs). CNTsuspension was prepared by ultrasonicating 1 mg of multiwalled CNT in 1mL DMF for an hour. The electrode was modified with CNT by drop casting16 μL (in 8 steps of 2 μL) followed by drying at 75° C. The electrodewas allowed to cool down on the ice before 2 μL of 10 mM PBSE in DMFwere added. The electrode was incubated on ice for 15 minutes to allowthe non-covalent binding between PBSE and CNT. DMF and 0.1 M PB pH 7.6were then used sequentially to remove the excessive PBSE from themodified electrode surface. A solution of TYR was prepared by dissolving5 mg TYR in 1 mL 20 mM PB pH 6.6 and a bi-enzyme solution mixture wasprepared by mixing 5 μL of salicylate hydroxylase solution and 5 μL TYRsolution. Bi-enzyme immobilized sensor was fabricated by drop casting 10μL of bi-enzyme solution on the electrode surface, and the electrode wasincubated on ice for 30 minutes to allow covalent bind of PBSE and bothenzymes. For control studies, mono-enzyme modified electrodes were alsofabricated with only one of the two enzymes, namely SH. The mono-enzymeelectrode was prepared by drop casting 5 μL of salicylate hydroxylasesolution. Electrode was rinsed with 0.1 M PB pH 7.6 to remove anyunimmobilized enzyme before measurement. For CV measurements, thepotential was scanned from 0.4 V to −0.2 V for bi-enzyme immobilizedelectrode and from −0.2 V to 0.4 V for both the unimmobilized andmono-enzyme modified electrodes. Scan rate of 20 mV s⁻¹ and sampleinterval of 0.001 V was applied for all CV experiments. The initialpotential for constant potential amperometry (CPA) with GC electrode wasset to 0.025 V with 0.1 s interval for data collection.

Results and Discussion Expression and Purification of SalicylateHydroxylase

Crude extract of SH enzyme synthesized from E. coli XL1-blue cells wascollected from the French Press after homogenization and centrifugation.The crude extract was first evaluated by traditional SH enzyme assaywith addition of FAD as cofactor (White-Stevens and Kamin 1972; Yamamotoet al. 1965). 4 mL of crude extract was obtained and the proteinconcentration of SH in the crude extract was determined by Bradfordassay to be ˜35 mg of total protein in 4 mL (Kruger 1994, The Bradfordmethod for protein quantitation. Basic protein and peptide protocols,pp. 9-15. Springer; and Kruger 2009, The Bradford method for proteinquantitation. The protein protocols handbook, pp. 17-24. Springer,incorporated by reference herein). Catalytic assay revealed that thetotal and specific activity of SH were ˜23 U and 0.67 U/mg of protein.After purification by fast protein liquid chromatography (FPLC) withHisTrap™ HP column, 0.73 mg of protein with the total activity of 8.96units was obtained. Although the purification yield was only 39%, thespecific activity of SH increased approximately 19 fold to ˜12.3 U/mg(Table 9).

Cyclic Voltammetry on Bi-Enzyme Modified Electrode

Control CV experiments were performed first using the mono-enzymebiosensor made of SH immobilized CNT electrodes. The study was carriedout by sequentially adding FAD, NADH and salicylate followed by CVmeasurement after each addition. 100 μL of 0.1 mM FAD was first addeddue to the requirement of FAD as the cofactor for the SH enzymereaction. The results of this CV experiment as shown in FIG. 23Ademonstrated that FAD does not show any electrochemical activity withinthe range of −0.2 V to 0.4 V and confirmed that any peak appeared in thesubsequent experiments were not that of FAD redox reactions. Then 50 μLof 10 mM NADH was added as the second cofactor and the electrochemicalredox activity of NADH was observed using CV between the same voltagewindow. The objective of this step is to reduce background currents fromNADH in the measurements. As shown in FIG. 23A, a small oxidation peakcan be found around 0.2 V due to the direct electrochemical oxidation ofNADH to NAD⁺. This corresponds with earlier reports of directelectrochemistry of NADH (Li et al. 2012a, NADH Oxidation catalyzed byelectropolymerized azines on carbon nanotube modified electrodes.Electroanalysis 24(2), 398-406; Li et al. 2012b, Quantitative Analysisof Bioactive NAD+ Regenerated by NADH Electro-oxidation. ACS Catalysis2(12), 2572-2576). In the next step, (sodium) salicylate was added tothe electrolyte to a final concentration of 25 μM and another CV scanwas performed. Under aerobic conditions, salicylate would bebiocatalytically reduced by SH to catechol while simultaneously NADH toNAD+(steps 1 and 2 in FIG. 22) as per the reaction below:

The resulting CV response is shown in FIG. 23A, where an oxidation peakat 0.15 V was observed upon the addition of 25 μM salicylate. This peakcan be attributed to the combined electrochemical oxidation of NADH andcatechol on the electrode surface. Catechol is electrochemicallyoxidized to 1,2-benzoquinone as per the following reaction at 0.15 V:

Catechol 1,2→Benzoquinone+2e ⁻+2H⁺

The reduction wave in all voltammograms below −0.1 V correspond to thereduction of dissolved oxygen present in the system. After understandingthe control response of SH immobilized mono-enzyme CNT electrode,similar set of experiments was performed using the bi-enzyme modifiedCNT electrode that contain both SH and TYR as recognition molecules. Thestudy was carried out by sequential addition of FAD, NADH and salicylatefollowed by CV measurement after each addition. Similar to the controlelectrode (mono-enzyme sensor), no significant oxidation/reduction peakcan be observed after adding FAD (FIG. 23B). The mild hump noticed at0.2 V in all the voltammograms in FIG. 23B is a characteristic of theblue copper proteins such as tyrosinase. However, unlike the SHmono-enzyme sensor, addition of NADH did not result in an oxidation peakin the case of bi-enzyme sensor (SH and TYR). This could be due to thereduced transport of NADH from the bulk to the electrode surface due tothe presence of additional protein in the CNT matrix on the modifiedelectrode. With addition of 25 μM salicylate, a prominent reduction peakappeared below 0.025 V as shown in FIG. 23B. This distinct reductionpeak appears only when both SH and TYR are present in the system andthus can be attributed to the direct electrochemical reduction of1,2-benzoquinone. 1,2-benzoquinone was produced by the biocatalyticoxidation of catechol by tyrosinase as per the following reaction:

Unlike the mono-enzyme SH electrode, the bi-enzyme electrode did notexhibit a direct electrochemical oxidation of the catechol in the 0.015V region of the anodic wave. This suggests that the biocatalyticoxidation reaction of catechol by TYR proceeds at a high rate depletingits surface concentration rather rapidly.

A further set of control experiments were performed on bothunimmobilized and TYR-immobilized mono-enzyme electrodes, both in thepresence and absence of catechol. The results of this experiment areshown in FIG. 24A, which clearly establishes the catechol redox peaks at˜0.15 V in the absence of TYR on the modified electrode (see indicatedcurve in FIG. 24A). Another set of control experiments were performed onboth unmodified and TYR modified mono-enzyme electrodes, both in thepresence and absence of salicylate, the result of which are shown inFIG. 24B. None of the voltammograms shown in FIG. 24B showed a directelectrochemical reduction of 1,2-benzoquinone as observed in FIG. 24B.This indicates that the 1,2-benzoquinone could only be generated in thesystem through the cascade reactions (steps 1 to 4), when both SH andTYR are present. The results provide conclusive evidence that thebi-enzyme sensor made of SH and TYR enzymes immobilized on CNT matrixprovide a reliable and selective detection of salicylate at potentialsbelow 0.15 V.

Determining Optimal Ratio of SH and TYR on Electrode Surface

The loading of either enzymes (SH or TYR) as well as the ratio of theirloadings on the CNT electrode surface could influence theelectrochemical detection and the resulting sensor performance. Thedifference in catalytic constants (K_(M) and k_(cat)) between the twoenzymes and the difference in mass transport coefficients of thereactants and products can be optimized for the cascade reactions (steps1 to 4 in FIG. 22). For example, if SH loading on the electrode isinsufficient, the cascadic reactions would be limited by catecholgeneration reaction, leading to low 1,2-benzoquinone generation and lowcurrents on the electrode, thereby directly impacting the sensitivity ofsalicylate detection. On the other hand, the cascade reactions will alsobe limited by step 3 (catechol to 1,2-benzoquinone conversion), if TYRloading is insufficient, which can also impact the selectivity ofdetection. It is desirable to optimize the kinetics and transport insidethe enzyme-CNT matrix of the bi-enzyme sensor for optimal conditions forreliable detection of salicylate.

An experimental design approach for fabricating bi-enzyme sensor withdifferent loadings of the two enzymes was prepared in this example. Forthis purpose, five different volume ratio of SH and TYR enzymes wereused for the immobilization on CNT electrode. The loading of SH and TYRused were: 1 μL and 9 μL, 3 μL and 7 μL, 5 μL and 5 μL, 7 μL and 3 μL,and 9 μL and 1 μL respectively on the electrode surface. CV wasperformed on the five bi-enzyme electrodes in the presence of the sameconcentration of salicylate, NADH, FAD and oxygen at the experimental pH7.6 and the results are shown in FIG. 25. The results show that rate of1,2-benzoquinone reduction (as determined by the slope of the reductionwave below 0.15 V) differed significantly when the enzyme loading ratiowas changed. The inset graph in FIG. 25, shows the current densityobserved at 0.1 V as a function of % volume of SH enzyme in the mixture,i.e. 50% refers to 1:1 volume loading of SH:TYR used for immobilization.The 0.1 V was outside both kinetic and mass transport limited regionsand therefore is an ideal reference point to measure the electrochemicalrate. It can also be noted from the insert graph in FIG. 25, that thecurrent for a 1:9 SH:TYR ratio was higher than that of 9:1 SH:TYR ratio.The trend indicates that the cascade reactions are limited by thereaction catalyzed by TYR (step 3) rather than the reaction catalyzed bySH (step 2). The highest sensitivity (current density) was observed fora SH:TYR volume ratio of 1:1. This corresponds to 1.83 μg of SH and 25μg of TYR on the electrode. Consequently, the mixture of 5 μL SH and 5μL TYR was used in all the remaining experiments to investigate thesensor performance characteristics such as sensitivity and limit ofdetection.

Electrochemical Response of the Bi-Enzyme Biosensor

Transient performance of the sensor was measured using the CV todetermine parameters such as sensitivity, LOD, limit of quantification(LOQ) and reliable linear range for salicylate detection. Since noelectrochemical peak can be observed by adding FAD and NADH (FIG. 23B),baseline was collected by CV after adding 100 μL of 0.1 mM FAD and 50 μLof 10 mM NADH. Then salicylate solution was added in steps to differentfinal concentrations 2.3 μM, 4.6 μM, 9.3 μM, 18.6 μM, 27.8 μM and 46.3μM and after each addition a CV was performed. The resultingvoltammograms shown in FIG. 26A indicated that the 1,2-benzoquinonereduction increased progressively (below 0.15 V) as the salicylateconcentration in the electrolyte was increased. The reduction currentsincreased up to 46.3 μM of salicylate beyond which the enzymes exhibitedsubstrate saturation. The effect of substrate limitation on the enzymekinetics can be explained by the Michaelis-Menten equation below:

V=V _(max) [S]/(K _(m) ,+[S])

As the concentration of substrate [S] is increased, the enzymaticreaction rate will eventually reach saturation and be equal to V_(max).The biosensor parameters were calculated from the CV data at 0.025 V asreference point. The insert graph in FIG. 26B shows the current densityat 0.025 V at different concentrations within the linear range ofdetection. The values were average of 3 replicates. From the data, thesensitivity was calculated to be 21.3±1.9 μA·cm⁻²·μM⁻¹ and the LOD andLOQ were determined to be 0.14±0.02 μM and 0.42±0.04 μM respectively.The linear range of salicylate detection using CV is 0 to 27.8 μM(R²=0.99) as listed in Table 10.

Since CV is a transient technique, it is usually used to obtain afirsthand understanding of the sensor, and a steady state measurement isobtained by constant potential amperometry (CPA). For the CPA, theinitial potential was set at 0.025 V and biosensor was stabilized for 2minutes before adding 100 μL of 0.1 mM FAD and 50 μL of 10 mM NADH at 1minute intervals, sequentially. After a 1 minute of preconditioning,salicylate was introduced stepwise in different quantities to finalconcentration of 2.3 μM, 4.6 μM, 9.3 μM, 18.6 μM, 27.8 μM to 46.3 μM.The reduction current was continuously monitored for 1 minute at eachconcentration until the next step addition of salicylate. For eachaddition of salicylate, the reduction current reached steady valuewithin short time and at high concentrations began to fade due to themass transfer limitations (FIG. 26B). Therefore, the highest currentmeasured at each concentration was used for calculating the sensorparameters, which are also reported in Table 10. Compared to the CV, thebi-enzyme biosensor exhibited higher sensitivity (30.6±2.7μA·cm⁻²·μM⁻¹), lower LOD (0.013±0.005 μM) and lower LOQ (0.039±0.015 μM)in the CPA measurements with the same linear range of salicylatedetection 0 to 27.8 μM (R²=0.99) (Table 10). Compared to the previouslydeveloped bi-enzyme methyl salicylate biosensor with alcohol oxidase andperoxidase, the sensitivity was successfully increased from 0.282μA·cm⁻²·μM⁻¹ to 30.6 μA·cm⁻²·μM⁻¹ and lowered the limit of detectionfrom 0.98 μM to 13 nM (Fang et al. 2016). The above parameters allows usto realize the quantification of MeSA produced by diseased plants inless than 3 minutes given that the produced MeSA is captured in 2 mLelectrochemical cell for detection based on the MeSA production rate of283 ng/plant/hr.

Evaluation of Reusability of the Bi-Enzyme Biosensor

It is desired that the biosensor is able to perform repeatedly duringmultiple measurements within a short period. To this end, reusability ofthe bi-enzyme biosensor was evaluated for 10 repetitions of salicylatedetection, and the results are shown in FIG. 27A. Similar to thesensitivity determination, salicylate solution was gradually added tothe electrolyte in the presence of FAD and NADH. The experiment wasrepeated 10 times and after each repetition, the electrode was taken outand rinsed to remove any residual catechol or 1,2-benzoquinone presenton the surface. Four salicylate concentrations within the linear range,namely 4.6, 9.3, 18.5 and 27.8 μM, were analyzed (10 repeatability testsfor each concentration, totaling 40 tests).

For low salicylate concentrations of 4.6 and 9.3 μM, the current keptincreasing during the first few repetitions (insert graph in FIG. 27A).This could be caused by the residues of 1.2-benzoquinone left on thesurface of the electrode from the previous repetition that was notremoved completely during rinsing. For low salicylate concentrations of4.6 and 9.3 μM, the current density remained constant at around 100% ofits original value, throughout the 10 repetitions with no obvious lossin sensitivity. On the other hand, at high salicylate concentrations of18.6 and 27.8 μM, a continuous loss in sensitivity was observed duringthe 10 repetitions. While the reason for the sensitivity loss duringrepeatable measurements at high salicylate concentration could beattributed to imbalance in the kinetics and mass transport at thesensor-electrolyte interface. However, detecting salicylateconcentrations above 10 μM are generally not necessary for earlydetermination of plant infections as the typical release rate of methylsalicylate by plants would fall below 10 μM (equivalent of 1.52 ppm).

Stability of the Bi-Enzyme Biosensor

In addition to reusability, stability of the biosensor was alsoevaluated using CPA technique. The bi-enzyme biosensor was fabricated onDay 1 and used to measure different concentrations of salicylate (4.6,9.3, 18.6 and 27.8 μM) on Day 1, using the previously describedexperimental procedure. After the experiments on Day 1, the biosensorwas rinsed by 0.1 M phosphate buffer (pH 7.6) and stored in 0.1 Mphosphate buffer (pH 7.6) with 10% glycerol at 4° C. The same sets ofexperiments were repeated on Day 2, 4, 6 and 8 and the current densitiesfor salicylate detection were monitored over time. The results of thesemeasurements are given in FIG. 27B. Similar to the results obtainedduring reusability evaluation, the currents at low salicylateconcentrations (4.6 and 9.3 μM) increased during the 2^(nd) measurement(on Day 2), likely due the residual catechol or 1,2-benzoquinone presenton the electrode that could not be removed during rinsing. The currentdensities for all other salicylate concentrations decreased graduallyafter Day 2 due to the gradual deterioration of enzymes on the sensorsurface (FIG. 27B). The currents took longer (10 to 60 second) to reachsteady values unlike on Day 1, where it reached steady values within 2seconds. Although long-term storage options and a suitable stabilizationmethod remain to be optimized, the results indicate that the bi-enzymesensor provides superior detection capabilities at early time periods.

Effect of Interference on the bi-Enzyme Biosensor

In addition to methyl salicylate, other volatile organic compounds(VOCs) can also be released by both healthy and stressed plants. Forexample, dichlorobenzene (DCB) and 1,2,3-trimethylbenzene (TMB) areamong the two most expressed VOCs released by healthy uninfectedsoybean. Farnesene (FAR) and humulene (HUM) are also reported to bereleased by soybean aphid infected soybean plant in addition to methylsalicylate. Therefore, the interference caused by FAR, HUM, DCB and TMBon amperometric detection of methyl salicylate using bi-enzyme sensorwere evaluated. Since the quantitative detection of methyl salicylatewas realized through salicylate measurements, samples have to behydrolyzed beforehand. Therefore, one potential interfering compoundthat is produced during this hydrolysis, methanol (MeOH), was alsoevaluated. In order to maintain the same ionic strength with 0.1 M PB(pH 7.6), 0.19 M KOH was used to hydrolyze the above-mentionedinterfering VOCs for 2 hrs in 90° C. water bath. Interfering VOC sampleswere prepared by adding phosphoric acid to adjust the pH to 7.6 beforeuse. CPA was used for interference evaluation in the presence of 100 μLof 0.1 mM FAD and 50 μL of 10 mM NADH. Very high concentrations (rangingfrom 9.3 μM to 1.9 mM) of MeOH, FAR, HUM, DCB and TMB were used for theinterference study. The upper range of 1.9 mM is 1000 fold higher thanthe typical VOCs concentration released by infected plants. This wasdone to ensure conservative estimate of interference under extreme(unfavorable) conditions. The experimental procedure used forinterference evaluation was similar to that of earlier CPA measurements.The results of these measurements are shown in FIG. 28. The resultsindicate that MeOH, HUM, DCB and TMB did not interfere significantlywith the salicylate detection current at the operating potential of0.025 V. Although FAR exhibits a noticeable interference (sensitivity of0.042 μA·cm⁻²·μM⁻¹), the currents are negligible compared to that of thecontrol electrode without interfering compounds (sensitivity of 30.61μA·cm⁻²·μM⁻¹) as shown in the insert graph in FIG. 28. It can beconcluded that none of the most common interfering compounds identifiedabove cause significant interference to the bi-enzyme sensor towards thedetection of salicylate of methyl salicylate.

Evaluation of Bi-Enzyme Biosensor Using Synthetic Analyte

Based on knowledge of plant volatile signatures, including thecomposition and relative molarity of the compounds that are released byuninfected and aphid-infected soybean plants, cocktails of VOCssimulating the healthy plant and infected soybean plant volatilesignatures were prepared and used as synthetic analyte to evaluate theperformance of the bi-enzyme sensor at near-practical conditions. Thecompositions of these synthetic analytes are given in Table 11. Thesynthetic analytes were prepared in 0.19 M KOH solution and hydrolyzedat 90° C. in a water bath for 2 hrs. Phosphoric acid was added to adjustpH to 7.6, before the synthetic analyte samples were used for biosensortests using CPA measurements. For the CPA measurement, the pH adjustedsynthetic analyte sample was gradually added to the electrolytecontaining 100 μL of 0.1 mM FAD and 50 μL of 10 mM NADH. The results ofthis measurement shown in FIG. 29 indicate the uninfected syntheticanalyte did not exhibit any noticeable reduction current even at highconcentrations of the synthetic analyte. On the other hand, for theinfected synthetic analyte, a stepwise increase in reduction currentswith concentration was observed. The qualitative and quantitative trendof aphid-infected analyte was nearly identical to that of the responsefrom pure salicylate as the analyte (control in FIG. 29). The measuredconcentration of MeSA in the synthetic analyte was calculated based onthe current versus concentration data given in FIG. 26B and the resultsare tabulated in Table 12. The ratio of the measured concentration tothe original concentration added was used to determine the recovery. Asshown in Table 12, most concentrations within the linear range exhibitsatisfactory recovery (close to 100%), suggesting reasonable sensoraccuracy for real sample measurement and quantification. The bi-enzymebiosensor exhibited a sensitivity of 33.49 μA·cm⁻²·μM⁻¹ for the infectedanalyte, which was not significantly different from that of thesensitivity obtained for pure methyl salicylate 30.61 μA·cm⁻²·μM⁻¹ asanalyte. This strongly suggests that the bi-enzyme biosensor can be usedfor reliable detection of real analyte released by infected crops.

Conclusions

A bi-enzyme based electrochemical biosensing platform includingsalicylate hydroxylase and tyrosinase as recognition elementsimmobilized onto a CNT matrix on the screen-printed carbon electrodesurface was constructed. The detection was based on a cascade of 4reaction steps that culminate in the electrochemical reduction of1,2-benzoquinone on the electrode. The fabricated biosensor wasevaluated for the selective detection of salicylate, a derivativecompound of methyl salicylate present in the volatile signature ofinfected crops. The bi-enzyme biosensor displayed high sensitivity andnano molar range for limit of detection. The sensor exhibited reasonablereusability and stability. The detection suffered very littleinterference from other common volatile organic compounds released byboth uninfected healthy plant and soybean-aphid infected plants.Synthetic analyte studies confirmed that the sensor can be used forreliable detection of real analytes of crop infection with highselectivity.

TABLE 9 Purification data of salicylate hydroxylase from E. coliXL1-blue Total Total Specific Purifi- Vol protein activity activitycation Yield Step (mL) (mg) (units) (units/mg) (fold) (%) Crude extract4 34.8 23.3 0.67  (1) (100) HisTrap ™ HP 2 0.73 8.96 12.34 19  39

TABLE 10 Sensor performance metrics for salicylate detection using CVand CPA techniques Linear Sensitivity range (μA · cm⁻² · LOD LOQ Method(μM) R² μM⁻¹) (μM) (μM) CV 0-27.8 0.99 21.3 ± 1.9 0.14 ± 0.01 0.42 ±0.04 CPA 0-27.8 0.99 30.6 ± 2.7 0.013 ± 0.005 0.039 ± 0.015

TABLE 11 Compositions of synthetic analyte simulating the VOC signatureof uninfected and soybean-aphid infected soybean plants Soybeanaphid-infected Uninfected synthetic analyte synthetic analyteConcentration Concentration voc (mM) VOC (mM) Dichlorobenzene 10 Methyl10 salicylate 1,2,3-trimethylbenzene 7 Farnesene 15 Humulene 10

TABLE 12 Simulated sample study with measuring simulated samples foruninfested, infested and salicylate Concentration Added ConcentrationMeasured Recovery (μM) (μM) (%) 4.6 4.73 102.77 9.3 10.15 109.11 18.622.76 122.34 27.8 28.93 104.06

Example 4—Detection of Methyl Salicylate on Tri-Enzyme ElectrochemicalSensor

This example describes an embodiment of a tri-enzyme functionalizedelectrochemical biosensor of the present disclosure with immobilizedsalicylate hydroxylase and tyrosinase for detection of methyl salicylatein combination with tannase or esterase as a third enzyme for hydrolysisof methyl salicylate.

The bi-enzyme systems described above rely on the manual hydrolysis ofmethyl salicylate (the target stress-induced plant volatile compound) tosalicylate and methanol. One of the hydrolysis products then acts as asubstrate for the next enzyme in the cascade ultimately producing anelectric current detectable at the electrode. However, use of an enzymefor the initial hydrolysis of methyl salicylate was investigated in thisexample for improvement in sensitivity and/or selectivity. FIG. 30provides a schematic illustration of a trienzymatic biosensor withtannase, and FIG. 31 illustrates a tri-enzyme system with esterase.

Experimental

Materials

Enzymes and other materials were obtained and/or prepared as describedin Example 1 and 3 above. Tannase from Aspergillus ficuum (powder, >150U/g) and esterase from porcine liver (lyophilized powder, >15 units/mgsolid) were purchased from Sigma and used as is. Apparatus forelectrochemical measurement and electrode preparation was as describedabove in Examples 1 and 3.

Results and Discussion

As illustrated in FIGS. 30 and 31, methyl salicylate (MeSA) produced bydiseased plants undergoes hydrolysis by tannase (FIG. 30) or esterase(FIG. 31) first to generate salicylate and methanol (1). The generatedsalicylate reacts on salicylate hydroxylase to produce catechol whileoxidizing NADH to NAD⁺ and reducing oxygen to water (2). The formedcatechol is further oxidized by tyrosinase and generate 1,2-benzoquinone(3). The detection of methyl salicylate is finally based on theelectrochemical reduction to regenerate catechol (4).

Cyclic voltammetry responses of methyl salicylate (MeSA) and salicylateon bi-enzyme and tri-enzyme systems were compared, as illustrated inFIGS. 32A-32D. CV responses of methyl salicylate (FIGS. 32A and 32B) andsalicylate (FIGS. 32C and 32D) on salicylate hydroxylase (SH) andtyrosinase (TRY) immobilized bienzymatic biosensor (FIGS. 32A and 32C)were compared to those of on esterase (ES), SH and TYR immobilizedtrienzymatic biosensors (FIGS. 32B and 32D). The results demonstratethat methyl salicylate cannot react on bienzymatic biosensor includingonly SH and TYR (FIG. 32A) (without manual hydrolysis of the methylsalicylate as described in the examples above). Significant reductioncurrent was observed from 0.15 V with methyl salicylate on trienzymaticbiosensors (FIG. 32B) which is similar to the same concentration ofsalicylate on both bienzymatic (FIG. 32C) and trienzymatic biosensor(FIG. 32D).

FIG. 33 demonstrates the cyclic voltammetry responses of 92 uM methylsalicylate on trienzymatic biosensor with different volume ratio ofesterase (5 mg/mL), salicylate hydroxylase and tyrosinase (5 mg/mL) andthe net current density against different esterase volume percentage(Insert). The experiment demonstrates that ratio of ES:SH:TYR=5:5:5generates the highest net current density.

As shown in FIG. 34, cyclic voltammetry (A) and constant potentialamperometry (B) responses of tri-enzymatic biosensor to differentconcentrations of methyl salicylate were evaluated. Inserted graphs arecurrent density versus concentration. Sensitivity was determined to be0.78 μA·cm⁻²·μM and 1.34 μA·cm⁻²·μM by CV and CPA, respectively

Conclusions

In addition to the SH and TYR-based bi-enzyme biosensor, a novel ES, SHand TYR-based trienzyme biosensor was successfully fabricated for MeSAdetection. The trienzyme biosensor does not require a manual hydrolysisof MeSA for the cascade reactions as implemented with the bi-enzymebiosensor. Therefore, a direct detection of MeSA released by diseasedplant or the disease-causing fungus can be realized through thetri-enzyme biosensor. The overall cascade reaction and the amperometricsignal generation (e.g., sensitivity) are limited by the catalyticactivity of the third enzyme, e.g., esterase (or tannase). This could beaddressed by employing a higher loading (g/cm2) of the third enzymeand/or by decreasing the diffusion path for MeSA to the enzyme on thesurface during its immobilization on the surface.

Example 5—Detection of p-ethylphenol on Tyrosinase Immobilized Biosensor

This example describes an embodiment of an enzyme functionalizedelectrochemical biosensor of the present disclosure with immobilizedtyrosinase for detection of p-ethylphenol. A characteristic VOC producedby strawberry plants during fungus infection such as P. cactorum isp-ethylphenol. Therefore, detection of p-ethylphenol produced bystrawberry plants in ultra-low quantities could be used as an effectiveindicator for crown rot infection. This example describes the successfuldevelopment of a tyrosinase-based enzymatic biosensor platform forselective detection of p-ethylphenol. Tyrosinase is an effective enzymethat catalyzes tyrosine, L-dopa and other o-diphenols to theircorresponding o-quinone derivatives. Additionally, tyrosinase is able tocatalyze monophenols to o-phenols (monooxygenase activity) and oxidizeo-phenols to corresponding o-quinones (catechol oxidase activity).

Tyrosinase was used as the bio-recognition element in the constructionof a biosensor. Tyrosinase biochemically oxidizes p-ethylphenol toproduce 4-ethyl-1,2-benzoquinone. The amperometric detection is based onthe electrochemical reduction of 4-ethyl-1,2-benzoquinone to4-ethyl-1,2-hydroquinone on a multiwalled carbon nanotube (CNT) modifiedelectrode. The scheme in FIG. 35 shows the mechanism of amperometricdetection of p-ethylphenol. Tyrosinase catalyzes the conversion ofp-ethylphenol to 4-ethyl-1,2-benzoquinone in the presence of oxygen.Tyrosinase was immobilized on CNT through a molecular tethering approachdescribed above, where the CNT acts as both immobilization support andconductive transducer.

Experimental

Materials

Tyrosinase (E.C. 1.14.18.1) from mushroom (lyophilized powder, 1000 U/mgsolid) was purchased from Sigma-Aldrich and used as received withoutfurther purification. Multiwalled carbon nanotubes (CNT) were obtainedfrom DropSens. Pyrenebutanoic acid succinimidyl ester (PBSE) waspurchased from AnaSpec Inc. (Fremont, Calif.). Dimethylformamide (DMF)and salicylate, sodium salt were used directly as received from AcrosOrganics. p-ethylphenol was obtained from Aldrich. Methanol waspurchased from Fisher Scientific. Ethanol was obtained from ElectronMicroscopy Sciences, Hatfield, Pa. Acetone was purchased from BDHchemicals. p-ethylguaiacol was obtained from Frinton Laboratories, Inc.,Hainesport, N.J. and used as directed. Ethyl butyrate and methylhexanoate were purchased from Fluka. Methyl butyrate, 2-pentanone and2-heptanone were obtained from Aldrich Chemicals. 0.1 M phosphate buffer(pH 6.6) was used as electrolyte for all experiments [25]. 18.2 MΩnano-pure de-ionized water was used for preparation of all solutions.Solutions were oxygenated by oxygen for 15 min prior to each experiment.

Apparatus

CH Instruments CHI 920c potentiostat was used to perform cyclicvoltammetry (CV), differential pulse voltammetry (DPV) and constantpotential amperometry. A three-electrode system having a 3 M Ag/AgCI asreference electrode, a platinum wire as counter electrode and aglassy-carbon (GC) electrode all obtained from Pine Instruments was usedto carry out the experiments in a custom made 5 mL glass voltammetrycell. All experiments were carried out at a temperature of 22±2° C.

Electrode Preparation

Glassy-carbon electrode was first polished with 0.05 micron aluminapower before each experiment. The electrode was then cleaned withultrasonication for 5 minutes and rinsed by DI water to remove aluminapower adhered to the electrode. The CNT suspension was prepared byultrasonicating 1 mg of CNT in 1 mL DMF for an hour. 16 μL of CNTsuspension was drop casted on the glassy-carbon electrode (in 8 steps of2 μL) followed by drying at 70° C. CNT modified electrode was placed inice bath to cool down, before adding 2 μL of PBSE in DMF on the CNTmodified electrode. The electrode was then incubated for 15 minutes toallow non-covalent binding of CNT with the pyrene group of PBSE. Theelectrode was rinsed with DMF to remove residual PBSE and then with 0.1M phosphate buffer (pH 6.6). Tyrosinase (TYR) solution was prepared bydissolving 5 mg of tyrosinase lyophilized powder in 1 mL of 0.1 Mphosphate buffer (pH 6.6). The electrode was further immobilized withtyrosinase by drop casting 5 μL of tyrosinase solution and incubated for30 minutes on ice for covalent binding of PBSE and the enzyme. Excesstyrosinase was removed by rinsing with 0.1 M phosphate buffer (pH 6.6).

Electrochemical Measurement

Cyclic voltammetry (CV) for unmodified CNT electrodes (no TYRimmobilized) was performed from 0.2 to 0.7 V with scan rate of 20 mV/sand sampling interval of 0.001 V. For the TYR -modified CNT electrodesthe range for CV was −0.2 to 0.4 V with scan rate of 20 mV/s andsampling interval of 0.001 V. Initial potential for unmodified CNT andTYR-modified CNT electrodes during constant potential amperometry wasset at 0.13 V with 0.1 s interval for data collection.

Results and Discussion

Determination of Voltage Window for Reliable Detection

Cyclic voltammetry was used to determine the potential window forreliable detection of p-ethylphenol through the electrochemicalreduction of 4-ethyl-1,2-benzoquinone (BQ) based on the reaction schemedescribed in FIG. 35. FIG. 36A shows the voltammograms of unmodified CNTelectrode between −0.2 and 0.7 V, in the presence and absence ofp-ethylphenol. The electrochemical oxidation of p-ethylphenol can benoticed above 0.45 V during the anodic sweep, while a correspondingreduction was absent. This suggests that p-ethylphenol oxidation at 0.4V is irreversible. Since the highly selective detection of p-ethylphenolcould only be realized based on the reaction scheme in FIG. 35, theelectrochemical oxidation of p-ethylphenol should be avoided when aTYR-modified electrode is used for the detection. Therefore, the voltagewindow was narrowed down to a shorter range from −0.2 to 0.4 V.

FIG. 36B shows the voltammograms of unmodified and TYR-modified CNTelectrodes both in the presence and absence of p-ethylphenol. Theresults demonstrate that p-ethylphenol cannot be detected in the voltagewindow between −0.2 and 0.4 V on the unmodified CNT electrode, since nosignificant oxidation or reduction peak could be observed within therange. On the other hand, upon the immobilization of TYR on CNT modifiedelectrode, the detection of p-ethylphenol was realized through thereduction of 4-ethyl-1,2-benzoquinone (BQ) to 4-ethyl-1,2-hydroquinone(HQ) below 0.2 V as per the scheme in FIG. 35. During the anodic sweep,as the potential was increased from −0.2 V to 0.1 V, two prominentoxidation peaks were also observed in FIG. 36B that could be attributedto the two-step electrochemical oxidation of 4-ethyl-1,2-hydroquinone(HQ).

Detection of p-ethylphenol Using Tyrosinase Modified CNT Electrode Thevoltammograms of TYR-modified CNT electrode at different concentrationsof p-ethylphenol is shown in FIG. 37A. The reduction peak at 0.13 V(conversion of 4-ethyl-1,2-benzoquinone to 4-ethyl-1,2-hydroquinone)started to increase with the concentration of p-ethylphenol from 0 to488 μM. The reduction currents appeared to reach saturation above 247 μMof p-ethylphenol, which is likely due to the saturation in enzymaticreaction rate V at high substrate concentrations [S] as explained by theMichaelis-Menten equation:

V=V _(max) [S]/(K _(m) +[S])

The reduction current at 0.13 V was plotted against the concentration ofp-ethylphenol in the insert of FIG. 37A. The dependence of the currenton concentration was linear. The sensitivity calculated as the slope ofthe insert graph is estimated to be 8.53 μA·cm⁻²·μM⁻¹. The TYR-modifiedCNT electrode also exhibited a limit of detection (LOD) of 0.21 μM andlimit of quantification (LOQ) of 0.64 μM for the detection ofp-ethylphenol (Table 13).

In addition to CV, constant potential amperometry (CPA) was also testedas described in the example above, to provide steady state measurementsto determine the sensitivity, LOD and LOQ for the analyte detection. CPAmeasurements were made using the tyrosinase-modified electrode at 0.13 V(the peak potential for 4-ethyl-1,2-benzoquinone reduction) by addingdifferent quantities of p-ethylphenol to result in a desired finalconcentration, while continuously monitoring the reduction current overtime. Electrodes were stabilized for 2 min before each addition ofp-ethylphenol, which was added to the electrolyte in the electrochemicalcell at 60-second intervals. The results of this measurement are shownin FIG. 37B. As concentration of p-ethylphenol increased, the reductioncurrent of 4-ethyl-1,2-benzoquinone also increased in all threerepetitive trials attempted for this measurement (FIG. 37B). Thesensitivity for p-ethylphenol detection was determined to be 4.05μA·cm⁻²·μM⁻¹ and the LOD and LOQ were determined to be 0.10 μM (12.2ppb) and 0.29 μM (35.4 ppb) respectively (Table 13).

The results indicate that the tyrosinase-modified electrode can bereliably used as a biosensor for the detection of p-ethylphenol in theconcentration range from 0 to 100 μM. In addition, the detection ofp-ethylphenol through the reduction of 4-ethyl-1,2-benzoquinone at lowpotentials (0.13 V) is advantageous compared to the detection throughdirect electrochemical oxidation of p-ethylphenol at 0.5 V (see FIG.36A), because the low potential detection, eliminates the interferenceother compounds typically present in the strawberry volatile signaturethat get oxidized at high potentials.

Stability of Tyrosinase Immobilized Biosensor

Constant potential amperometry was also used to evaluate the stabilityof the biosensor. For this, the tyrosinase-modified electrode wasfabricated on Day 1 and its sensitivity towards p-ethylphenol detectionwas determined using the same procedure explained above betweenconcentrations 0 and 100 μM. After this, the electrode was rinsed with0.1 M phosphate buffer (pH 6.6) and stored in 20 mM phosphate buffer (pH6.6) with 10% glycerol at 4° C. The same experiment was repeated on Day2, Day 4, Day 6, Day 8, Day 10 and Day 12. Current densities atp-ethylphenol concentrations of 10, 25, 50 and 100 μM were compared inFIG. 38. As the result show, the current density started to decreasefrom Day 2 for all four concentrations (FIG. 38). This could beattributed to the accelerated degradation of the enzyme's activity onthe electrode surface over time. About 50% of the current density wasretained on Day 8.

Biosensor Performance in the Presence of Interference Compounds

During a biotic stress event such as during P. cactorum infection,strawberry plants produce p-ethylphenol in high quantities. However anuninfected healthy strawberry plant also produces and releases a varietyof other volatile organic compounds. Compounds such as ethyl butanoate,methyl hexanoate, ethanol, acetone, methyl butanoate, 2-heptanone and2-pentanone are mostly produced as volatile organic compounds by healthystrawberry plants at all times. The interference from these compounds tothe amperometric signal for p-ethylphenol detection is evaluated toavoid false positive detection. Different concentrations of theabove-mentioned interfering compounds up to 6.67 mM were analyzed andtheir interfering currents were compared against the amperometric signalof 10 μM p-ethylphenol. The results of the CPA measurements are shown inFIG. 39. The values for interference currents obtained from the CPAmeasurements and the corresponding concentration (in both mM and ppm)are tabulated in Table 14. Among all the interference compounds testedin different concentration, only 6.67 mM of 2-heptanone produced morethan 3% of current density compared to 10 μM p-ethylphenol. The resultsprove that none of the interference compound listed above results insignificant interference, although some current density peaks are stillpresent due to the disturbance of sample mixing (FIG. 39).

In addition to the interference compounds listed above, p-ethylguaiacolis a volatile organic compound produced simultaneously withp-ethylphenol when strawberry is infected by P. cactorum.p-ethylguaiacol can also be used as a marker for leather rot diseasedetection. Since p-ethylguaiacol is also released along withp-ethylphenol by the infected strawberry plant, the interference ofp-ethylguaiacol on p-ethylphenol detection signal was evaluated asdescribed above, and the results are provided in Table 15. Compared toother compounds, p-ethylguaiacol showed noticeable interference of 9.0%at 250 μM (Table 14). However in typical plant volatile signatures, thep-ethylguaiacol is released in much smaller quantities compared top-ethylphenol and therefore is not a cause for concern for thisbiosensor.

Evaluation of Biosensor Using Synthetic Analyte Cocktail

In order to evaluate the sensor in near-practical conditions, asynthetic blend (cocktail) of volatile compounds was used as the analyteto mimic the production of volatile organic compounds by healthystrawberries. The cocktail was prepared using 7 compounds atcompositions similar to that in the volatile signature of strawberriesas listed in Table 15. Experiments were performed with addition of 1 mLsynthetic blend (cocktail) added to 1 mL 20 mM phosphate buffer at 120seconds to mimic the volatile signature of a healthy strawberry plant.The mixed solution of 20 mM p-ethylphenol and 20 mM p-ethylguaiacol wasthen gradually added at 60-second intervals to mimic the release ofp-ethylphenol along with p-ethylguaiacol by infected strawberries inaddition to the volatile organic compounds released by the healthystrawberries (synthetic cocktail). The results from the experiments werecompared with the control experiment performed by adding pure 20 mMp-ethylphenol without the presence of interference compounds. Theresults indicate the trend in currents for p-ethylphenol in presence ofcocktail was similar to that of the control group that did not containinterference compounds (FIG. 40). In addition, the sensitivity ofp-ethylphenol detection for the p-ethylphenol in presence of interferingcompounds from cocktail was similar to that of pure p-ethylphenolwithout any interfering compound. The results demonstrate that thebiosensor could be used for reliable detection of p-ethylphenol from areal plant volatile signature with little interference from itsconstituent compounds

Conclusions

The CNT based enzymatic biosensor described in this example exhibitedhigh sensitivity, ultra-low detection, and quantification limits for thedetection of p-ethylphenol. The biosensor also displayed satisfactorystability. Other volatile organic compounds have been tested forinterference, and no significant interference for p-ethylphenoldetection was exhibited by any of the compounds tested. Syntheticanalyte including p-ethylphenol and other typical volatile organiccompounds produced by both healthy and unhealthy strawberry plants wasused for evaluating the sensor under near-practical conditions, and thesensor exhibited reliable detection of p-ethylphenol in the syntheticanalyte. This research provides a platform for the development ofbiosensors for early detection of plant diseases and has significantimplications in the field of agriculture.

TABLE 13 Linear range, R² value, sensitivity, limit of detection (LOD)and limit of quantification (LOQ) for tyrosinase-modified CNT biosensorfor p-ethylphenol detection by cyclic voltammetry (CV) and constantpotential amperometry (CPA) Linear Tech- range Sensitivity nique (μM) R²(μA · cm² · μM⁻¹) LOD (μM) LOQ (μM) CV 0-100 0.9950 8.53 ± 0.95 0.21 ±0.08 0.64 ± 0.25 CPA 0-100 0.9956 4.05 ± 0.52 0.10 ± 0.02 0.29 ± 0.07

TABLE 14 Percentage (%) of interference current density resulted frominterference compounds compared to 10 μM p-ethylphenol Conc. (mM) EB MHAc Et MB HN PN EG 0   0 ppm   0 ppm   0 ppm   0 ppm   0 ppm   0 ppm   0ppm   0 ppm 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.0% 0.05  5.8 ppm  6.5ppm 2.9 ppm 2.3 ppm  5.1 ppm  5.7 ppm 4.3 ppm  7.6 ppm 0.7% 1.2% 0.3%0.2% 0.4% 0.7% 0.4% 1.4% 0.10 11.6 ppm 13.0 ppm 5.8 ppm 4.6 ppm 10.2 ppm11.4 ppm 8.6 ppm 15.2 ppm 0.0% 0.2% −0.1%   −0.2%   −0.2%   0.4% 0.4%3.6% 0.25 29.0 ppm 32.5 ppm 14.5 ppm  11.5 ppm  25.5 ppm 28.5 ppm 21.5ppm  38.0 ppm 0.8% 0.7% 0.6% 0.9% 0.4% 0.7% 0.5% 9.0% *EB: ethylbutyrate; MH: methyl hexanoate; Ac: acetone; Et: ethanol; MB: methylbutyrate; HN: 2-heptanone; PN: 2-pentanone; EG: p-ethylguaiacol

TABLE 15 Composition of synthetic analyte cocktail including volatileorganic compounds (VOCs) produced by healthy strawberry plants Volatileorganic compound Concentration (mM) Ethyl butanoate 20.75 Methylhexanoate 16.62 Acetone 7.42 Ethanol 7.42 Methyl butanoate 13.912-Heptanone 9.72 2-Pentanone 3.91

REFERENCES

-   Armstrong, N. R.; Lin, A. W. C.; Fujihira, M.; and Kuwana, T.; Anal    Chem, 1976, 48, 741.-   Atanassov, P.; Ivnitski, D.; Ramasamy, R. P.; Luckarift, H. R.;    Johnson, G. R.; Lau, C. Biofuel cell electrocatalysts using    enzyme-carbon nanotube adducts. 2014.-   Bakker, E., Electrochemical sensors. Analytical chemistry, 2004.    76(12): p. 3285-3298.-   Baldwin, I. T.; Kessler, A.; and Halitschke, R.; Curr Opin Plant    Biol., 2002, 5, 351.-   Berna A. Z.; Lammertyn J.; Saevels S.; Natale C. D. and NicolaI B.    M.; Sensor Actuat B-Chem., 2004, 97, 324.-   Bru, R., A. Sanchez-Ferrer, and F. Garcia-Carmona, Characteristics    of tyrosinase in AOT—isooctane reverse micelles. Biotechnology and    bioengineering, 1989. 34(3): p. 304-308.-   Buttery, R., Seifert, R., Guadagni, D., Ling, L., 1969.    Characterization of some volatile constituents of bell peppers.    Journal of Agricultural and Food Chemistry 17(6), 1322-1327.-   Caboni P.; Sarais G.; Cabras M. and Angioni A.; J Agr Food Chem.,    2007, 55, 7288.-   Calkins, J. O.; Umasankar, Y.; O′Neill, H.; Ramasamy, R. P., High    photo-electrochemicalactivity of thylakoid-carbon nanotube    composites for photosynthetic energy conversion. Energ Environ Sci    2013, 6, 1891-1900.-   Comninellis, C.; and Pulgarin, C.; J Appl Electrochem., 1993, 23,    108.-   Cosnier, S. and C. Innocent, A new strategy for the construction of    a tyrosinase-based amperometric phenol and o-diphenol sensor.    Bioelectrochemistry and bioenergetics, 1993. 31(2): p. 147-160.-   Costello, B. P. J. d. L.; Ewen, R. J.; Jones, P. R. H.;    Ratcliffe, N. M.; Wat, R. K. M., A study of the catalytic and    vapour-sensing properties of zinc oxide and tin dioxide in relation    to 1-butanol and dimethyldisulphide. Sensors and Actuators B 1999,    61, 199-207.-   de Faria, R.O., et al., The tyrosinase produced by Lentinula boryana    (Berk & Mont.) Pegler suffers substrate inhibition by L-DOPA. Food    Technology and Biotechnology, 2007. 45(3): p. 334-340.-   Diebold, U.; Appl Phys A-Mater., 2003, 76, 681.-   Ellis, M. A. and Grove, G. G., Leather rot in Ohio strawberries.    Plant disease, 1983. 67(5).-   Enache, T. A. and A. M. Oliveira-Brett, Phenol and para-substituted    phenols electrochemical oxidation pathways. Journal of    Electroanalytical Chemistry, 2011. 655(1): p. 9-16.-   Espin, J. C., et al., Kinetic characterization of the substrate    specificity and mechanism of mushroom tyrosinase. European Journal    of Biochemistry, 2000. 267(5): p. 1270-1279.-   Fang, Y., Ramasamy, R. P., 2015. Current and Prospective Methods for    Plant Disease Detection. Biosensors 5(3), 537-561.-   Fang, Y., Umansankar, Y., and Ramasamy, R. P., Electrochemical    detection of p-ehtylguaiacol, a fungi infected fruit volatile using    metal oxide nanoparticles. Analyst, 2014, 139, 3804.-   Fang, Y., Y. Umasankar, and R. P. Ramasamy, A novel bi-enzyme    electrochemical biosensor for selective and sensitive determination    of methyl salicylate. Biosensors and Bioelectronics, 2016.-   Farina, L., et al., Determination of volatile phenols in red wines    by dispersive liquid-liquid microextraction and gas    chromatography-mass spectrometry detection. Journal of    Chromatography A, 2007. 1157(1): p. 46-50.-   Godfray, H. C. J., Beddington, J. R., Crute, I. R., Haddad, L.,    Lawrence, D., Muir, J. F., Pretty, J., Robinson, S., Thomas, S. M.,    Toulmin, C., 2010. Food security: the challenge of feeding 9 billion    people. science 327(5967), 812-818.-   Hakala, M. A., A. T. Lapvetelainen, and H. P. Kallio, Volatile    compounds of selected strawberry varieties analyzed by    purge-and-trap headspace GC-MS. Journal of agricultural and food    chemistry, 2002. 50(5): p. 1133-1142.-   Ingram, J., 2011. A food systems approach to researching food    security and its interactions with global environmental change. Food    Security 3(4), 417-431.-   Jeleń, H.; Krawczyk, J.; Larsen, T.; Jarosz, A.; and Golebniak, B.;    Main compounds responsible for off-odour of strawberries infected by    Phytophthora cactorum. Letters in applied microbiology, 2005.    40(4): p. 255-259.-   Katagiri, M., Maeno, H., Yamamoto, S., Hayaishi, O., Kitao, T., Oae,    S., 1965. Salicylate hydroxylase, a monooxygenase requiring flavin    adenine dinucleotide II. The mechanism of salicylate hydroxylation    to catechol. Journal of Biological Chemistry 240(8), 3414-3417.-   Keinan, A., Clark, A. G., 2012. Recent explosive human population    growth has resulted in an excess of rare genetic variants. science    336(6082), 740-743.-   Kessler, A., Baldwin, I. T., 2001. Defensive function of    herbivore-induced plant volatile emissions in nature. Science    291(5511), 2141-2144.-   Kohlmann, C.; Robertz, N.; Leuchs, S.; Dogan, Z.; Lutz, S.; Bitzer,    K.; Na'amnieh, S.; Greiner, L., Ionic liquid facilitates    biocatalytic conversion of hardly water soluble ketones. J Mol Catal    B-Enzym 2011, 68, 147-153.-   Kounaves, S. P.; in Handbook of Instrumental Techniques for    analytical Chemistry, ed. A. A. Settle, Prentice Hall PTR, Upper    Saddle River, N.J., 1997, Ch 37, pp. 709-726.-   Kruger, N. J., 1994. The Bradford method for protein quantitation.    Basic protein and peptide protocols, pp. 9-15. Springer.-   Laothawornkitkul, J., Moore, J. P., Taylor, J. E., Possell, M.,    Gibson, T. D., Hewitt, C. N., Paul, N. D., 2008. Discrimination of    plant volatile signatures by an electronic nose: a potential    technology for plant pest and disease monitoring. Environmental    science & technology 42(22), 8433-8439.-   Li, H., Wen, H., Calabrese Barton, S., 2012a. NADH Oxidation    catalyzed by electropolymerized azines on carbon nanotube modified    electrodes. Electroanalysis 24(2), 398-406.-   Li, H., Worley, K. E., Calabrese Barton, S., 2012b. Quantitative    Analysis of Bioactive NAD+ Regenerated by NADH Electro-oxidation.    ACS Catalysis 2(12), 2572-2576.-   Lian, P.; Zhu, X.; Liang, S.; Li, Z.; Yang, W.; and Wang, H.;    Electrochimica Acta, 2011, 56, 4532.-   Loake, G., Grant, M., 2007. Salicylic acid in plant defence—the    players and protagonists. Current opinion in plant biology 10(5),    466-472.-   Luo, P., S. V. Prabhu, and R. P. Baldwin, Constant potential    amperometric detection at a copper-based electrode: electrode    formation and operation. Analytical Chemistry, 1990. 62(7): p.    752-755.-   Mahlein, A.-K., Oerke, E.-C., Steiner, U., Dehne, H.-W., 2012.    Recent advances in sensing plant diseases for precision crop    protection. European Journal of Plant Pathology 133(1), 197-209.-   Martorell, N., et al., Determination of 4-ethylguaiacol and    4-ethylphenol in red wines using headspace-solid-phase    microextraction-gas chromatography. Journal of Chromatography    A, 2002. 975(2): p. 349-354.-   Nam, S.; Jeon, H.; Kim, S. H.; Jang, J.; Yang, C.; Park, C. E., An    inkjet-printed passivation layer based on a photocrosslinkable    polymer for long-term stable pentacene field-effect transistors. Org    Electron 2009, 10, 67-72-   Natale C. D.; Salimbeni D.; Paolesse R.; Macagnano A.; D'Amico A.;    Sensor Actuat B-Chem., 2000, 65, 220.-   Nieminen, T., et al., Volatile compounds produced by fungi grown in    strawberry jam. LWT-Food Science and Technology, 2008. 41(10): p.    2051-2056.-   Paré P. W. and Tumlinson J. H.; Plant Pysiol., 1999, 121, 325.-   Parimi, N. S.; Umasankar, Y.; Atanassov, P.; Ramasamy, R. P.,    Kinetic and Mechanistic Parameters of Laccase Catalyzed Direct    Electrochemical Oxygen Reduction Reaction. Acs Catal 2012, 2, 38-44.-   Park, S., H. Boo, and T. D. Chung, Electrochemical non-enzymatic    glucose sensors. Analytica Chimica Acta, 2006. 556(1): p. 46-57.-   Pawliszyn, J., 1999. Applications of solid phase microextraction.    Royal Society of Chemistry.-   Persaud K. and Dodd G., Nature., 1982, 299, 352.-   Pichersky, E., Noel, J. P., Dudareva, N., 2006. Biosynthesis of    plant volatiles: nature's diversity and ingenuity. Science    311(5762), 808-811.-   Pollnitz, A. P., K. H. Pardon, and M. A. Sefton, Quantitative    analysis of 4-ethylphenol and 4-ethylguaiacol in red wine. Journal    of Chromatography A, 2000. 874(1): p. 101-109.-   Ramasamy, R. P.; Luckarift, H. R.; Ivnitski, D. M.; Atanassov, P.    B.; Johnson, G. R., High electrocatalytic activity of tethered    multicopper oxidase-carbon nanotube conjugates. Chem Commun 2010,    46, 6045-6047.-   Rassaei, L., et al., Substrate-dependent kinetics in    tyrosinase-based biosensing: amperometry vs. spectrophotometry.    Analytical and bioanalytical chemistry, 2012. 403(6): p. 1577-1584.-   Rayne S. and Eggers N. J.; J Environ Sci Heal B., 2007, 42, 887.-   Roosen, C.; Muller, P.; Greiner, L., Ionic liquids in biotechnology:    applications and perspectives for biotransformations. Appl Microbiol    Biot 2008, 81, 607-614.-   Sadana, A.; and Katzer, J. R.; J Catal., 1974, 35, 140.-   Saevels S.; Lammertyn J.; Berna A. Z.; Veraverbeke E. A.;    Natale C. D. and Nicolaï B. M.; Postharvest Biol Tec., 2004, 31, 9.-   Sankaran, S., Mishra, A., Ehsani, R., Davis, C., 2010. A review of    advanced techniques for detecting plant diseases. Computers and    Electronics in Agriculture 72(1), 1-13.-   Schmelz, E. A., Engelberth, J., Alborn, H. T., O'Donnell, P.,    Sammons, M., Toshima, H., Tumlinson, J. H., 2003. Simultaneous    analysis of phytohormones, phytotoxins, and volatile organic    compounds in plants. Proceedings of the National Academy of Sciences    100(18), 10552-10557.-   Seskar, M., Shulaev, V., Raskin, I., 1998. Endogenous methyl    salicylate in pathogen-inoculated tobacco plants. Plant Physiology    116(1), 387-392.-   Shulaev, V., Silverman, P., Raskin, I., 1997. Airborne signalling by    methyl salicylate in plant pathogen resistance.[Erratum: Apr 17,    1997, v. 386 (6626), p. 738.]. Nature.-   Smil, V., 2001. Feeding the world: A challenge for the twenty-first    century. MIT press.-   Suneesh P. V.; Chandhini K.; Ramachandran T.; Nair B. G.; Satheesh    Babu T. G., Biosens Bioelectron., 2013, 50, 472.-   Sunesson, A.; Vaes, W.; Nilsson, C.; Blomquist, G.; Andersson, B.;    and Carlson, R.; Appl Environ Microb., 1995, 61, 2911.-   Ubeda, C., et al., Characterization of odour active compounds in    strawberry vinegars. Flavour and Fragrance Journal, 2012. 27(4): p.    313-321.-   Ubeda, C., et al., Glycosidically bound aroma compounds and impact    odorants of four strawberry varieties. Journal of agricultural and    food chemistry, 2012. 60(24): p. 6095-6102.-   Umasankar, Y., et al., Three Dimensional Carbon Nanosheets as a    Novel Catalyst Support for Enzymatic Bioelectrodes. Advanced Energy    Materials, 2014. 4(6).-   Umasankar, Y.; Rains, G. C.; Ramasamy, R. P., Electroanalytical    studies on green leaf volatiles for potential sensor development.    Analyst 2012, 137, 3138-3145.-   Umasankar, Y.; Ramasamy, R. P., Catalytic activity of tyrosinase for    potential biofuel cell application. ECS Transactions 2013, 45, 9-14.-   Umasankar, Y.; Ramasamy, R. P., Highly sensitive electrochemical    detection of methyl salicylate using electroactive gold    nanoparticles. Analyst 2013, 138, 6623-6631.-   Umasankar, Y.; Ramasamy, R. P., On the bio-electrocatalytic activity    of tyrosinase for oxygen reduction reaction. Catal Sci Technol 2013,    3, 2546-2549.-   van Rantwijk, F.; Sheldon, R. A., Biocatalysis in ionic liquids.    Chem Rev 2007, 107, 2757-2785.-   Weetall, H. H. and T. Hotaling, A simple, inexpensive, disposable    electrochemical sensor for clinical and immuno-assay.    Biosensors, 1988. 3(1): p. 57-63.-   White-Stevens, R. H., Kamin, H., 1972. Studies of a flavoprotein,    salicylate hydroxylase I. Preparation, properties, and the    uncoupling of oxygen reduction from hydroxylation. Journal of    Biological Chemistry 247(8), 2358-2370.-   Wolcott, A.; Smith, W. A.; Kuykendall, T. R.; Zhao, Y. P.; Zhang, J.    Z., Photoelectrochemical Water Splitting Using Dense and Aligned    TiO2 Nanorod Arrays. Small 2009, 5, 104-111.-   Yamamoto, S., Katagiri, M., Maeno, H., Hayaishi, O., 1965.    Salicylate hydroxylase, a monooxygenase requiring flavin adenine    dinucleotide I. Purification and general properties. Journal of    Biological Chemistry 240(8), 3408-3413.-   You, I. S., Ghosal, D., Gunsalus, I. C., 1991. Nucleotide sequence    analysis of the Pseudomonas putida PpG7 salicylate hydroxylase gene    (nahG) and its 3′-flanking region. Biochemistry 30(6), 1635-1641.-   You, I.-S., Murray, R., Jollie, D., Gunsalus, I., 1990. Purification    and characterization of salicylate hydroxylase from Pseudomonas    putida PpG7. Biochemical and biophysical research communications    169(3), 1049-1054.-   Zhang, Z., Pawliszyn, J., 1993. Headspace solid-phase    microextraction. Analytical chemistry 65(14), 1843-1852.-   Zhou, Y.; Umasankar, Y.; Ramasamy, R. P., Enzyme—metal oxide    composites as catalysts for enzymatic oxygen reduction reaction. In    225th Electrochemical Society Meeting, Orlando, 2014.-   Zhou, Y.; Umasankar, Y.; Ramasamy, R. P., Laccase-TiO2    nanoconjugates as catalysts for bio-electrochemical oxygen reduction    reaction. Acs Catal 2014, in review.-   Zhu, J., Park, K.-C., 2005. Methyl salicylate, a soybean    aphid-induced plant volatile attractive to the predator Coccinella    septempunctata. Journal of chemical ecology 31(8), 1733-1746.-   Zhuang Z.; Su X.; Yuan' H.; Sun, Q.; Xiao, D.; and Choi, M. M. F.;    Analyst., 2008, 133, 126.

We claim:
 1. An electrochemical sensor comprising a volatile detectionelectrode comprising: an electrode substrate; and a bio-nanocompositedetection element on a surface of the electrode substrate and inelectrochemical communication with the electrode substrate, thebio-nanocomposite detection element comprising: a nanomaterialtransducer material, and an enzyme system capable of specific reactionwith a target stress-induced plant volatile compound methyl salicylate,its hydrolysis product, or both, wherein the enzyme system comprises abi-enzyme or tri-enzyme system comprising at least an enzyme pairselected from the group of enzyme pairs consisting of: salicylatehydroxylase and tyrosinase, and alcohol oxidase and horseradishperoxidase, wherein the enzymes are immobilized on the nanomaterialtransducer material, and reaction between the enzymes and methylsalicylate, its hydrolysis product, or both generates an electricalsignal, wherein detection of the electrical signal indicates thepresence of methyl salicylate.
 2. The electrochemical sensor of claim 1,further comprising a detection mechanism for detecting the electricalsignal generated at the detection electrode when the enzymes react withthe methyl salicylate, its hydrolysis product, or both.
 3. Theelectrochemical sensor of claim 1, wherein the enzyme pair comprisessalicylate hydroxylase and tyrosinase.
 4. The electrochemical sensor ofclaim 1, wherein the enzyme system comprises a tri-enzyme systemcomprising a first enzyme and the enzyme pair, the first enzyme capableof hydrolyzing methyl salicylate to salicylate and methanol, such thateach enzyme in the enzyme pair reacts with the salicylate or methanol ina cascade of reactions, wherein the final reaction in the cascadegenerates the electrical signal.
 5. The electrochemical sensor of claim4, wherein the first enzyme is selected from the group consisting of:tannase and esterase.
 6. The electrochemical sensor of claim 1, whereinthe bio-nanocomposite detection element further comprises one or moreenzymes capable of specific reaction with a second target volatilecompound selected from a target stress-induced plant volatile compoundor a target pathogen-emitted volatile compound.
 7. The electrochemicalsensor of claim 6, wherein second target volatile compound is selectedfrom the group consisting of: ethyl phenol, ethyl guaiacol, octanone,octanol, a green leaf volatile compound, and combinations of thesevolatile compounds.
 8. The electrochemical sensor of claim 6, whereinthe one or more enzymes capable of specific reaction with a secondtarget volatile compound is selected from the group consisting of:tyrosinase (TYR), laccase (Lc), bilirubin oxidase (BRO), horseradishperoxidase (HRP), salicylate hydroxylase, alcohol oxidase (AO), alcoholdehydrogenase (ADH), and combinations of these enzymes.
 9. Theelectrochemical sensor of claim 7, wherein the second target volatilecompound is selected from ethyl phenol, ethyl guaiacol, or both, and theenzyme capable of reaction with the second target volatile compound isselected from horseradish peroxidase or tyrosinase.
 10. Theelectrochemical sensor of claim 7, wherein the second target volatilecompound is octanone and the enzyme capable of detecting octanone isADH.
 11. The electrochemical sensor of claim 1, further comprising asecond volatile detection electrode that detects at least a secondtarget volatile compound other than methyl salicylate, the secondvolatile detection electrode comprising: an electrode substrate; and abio-nanocomposite detection element on a surface of the electrodesubstrate and in electrochemical communication with the electrodesubstrate, the bio-nanocomposite detection element comprising: ananomaterial transducer material, and one or more enzymes capable ofspecific reaction with the second target volatile compound or itshydrolysis product, wherein the second target volatile compound is astress-induced plant volatile compound or a target pathogen-emittedvolatile compound, wherein the one or more enzymes are immobilized onthe nanomaterial transducer material and reaction between the one ormore enzyme and the second target volatile compound generates anelectrical signal, wherein detection of the electrical signal indicatesthe presence of the second target volatile compound.
 12. Theelectrochemical sensor of claim 11, wherein the second target volatilecompound is selected from the group consisting of: ethyl phenol, ethylguaiacol, octanone, octanol, a green leaf volatile compound, andcombinations of these volatile compounds, and wherein the one or moreenzymes capable of specific reaction with the second target volatilecompound is selected from the group consisting of: tyrosinase (TYR),laccase (Lc), bilirubin oxidase (BRO), horseradish peroxidase (HRP),salicylate hydroxylase, alcohol oxidase (AO), alcohol dehydrogenase(ADH), and combinations of these enzymes.
 13. The electrochemical sensorof claim 1, wherein the nanomaterial transducer material comprises ananomaterial selected from the group consisting of: multiwalled carbonnanotubes (MWCNTs), carbon nanoparticles, graphene nanoparticles, metalnanoparticles, and metal oxide nanoparticles.
 14. The electrochemicalsensor of claim 1, wherein the nanomaterial transducer materialcomprises multiwalled carbon nanotubes (MWCNTs), and the one or moreenzymes are immobilized to the MWCNTs via a tethering agent selectedfrom the group consisting of: a 1-pyrene butanoic acid succinamidylester (PBSE) tethering agent,4,4′-[(8,16-dihydro-8,16-dioxodibenzo[a,j]perylene-2,10-diyl)dioxy]dibutyric acid di(N-succinimidyl ester) (DPPSE) tethering agent, andother PBSE-type heterobifunctional tethering agents.
 15. Theelectrochemical sensor of claim 1, wherein the one or more enzymes areimmobilized to the nanomaterial transducer material via a linking groupcomprising an amine-terminated silane cross-linker attached to thenanomaterial and an amine-amine crosslinker attached to an amine of theamine-terminated silane cross-linker.
 16. The electrochemical sensor ofclaim 1, wherein the volatile detection electrode is a working electrodeof an electrochemical cell further comprising a counter electrode and areference electrode in electrochemical communication with the workingelectrode, and a potentiostat configured to supply an electric currentto the electrochemical cell and monitor changes in the electric currentgenerated at the working electrode.
 17. The electrochemical sensor ofclaim 16, wherein changes in the electric current in the electrochemicalcell are recorded as a cyclic voltammogram, differential pulsevoltammogram, or other current response to an applied potential orvoltage.
 18. The electrochemical sensor of claim 1, further comprising amediator to facilitate electron transfer from the enzyme to theelectrode.
 19. A plant volatile detection system comprising: (a) avolatile collection reservoir adapted to collect volatile compoundsemitted from a plant; (b) an electrochemical sensor of claim 1 furthercomprising a counter electrode, and a reference electrode, wherein boththe counter electrode and the reference electrode are in electrochemicalcommunication with the volatile detection electrode, and a potentiostatto supply an electric current to the electrochemical cell and monitorchanges in the electric current produced at the volatile detectionelectrode; and (c) a signal processing mechanism in operativecommunication with one or more elements of the electrochemical sensor,the signal processing mechanism having data transfer and evaluationsoftware protocols configured to transform raw data from theelectrochemical sensor into diagnostic information regarding thepresence or absence or levels of the target plant volatile compound. 20.A method for monitoring a condition of a plant or crop of plants, themethod comprising: (1) periodically sampling volatile emissions from theplant or one or more crop plants using a plant volatile detectionsystem, the plant volatile detection system comprising: (a) a volatilecollection reservoir adapted to collect volatile compounds emitted froma plant; (b) an electrochemical sensor of claim 1 further comprising acounter electrode, and a reference electrode, wherein both the counterelectrode and the reference electrode are in electrochemicalcommunication with the volatile detection electrode, and a potentiostatto supply an electric current to the electrochemical cell and monitorchanges in the electric current produced at the volatile detectionelectrode; and (c) a signal processing mechanism in operativecommunication with one or more elements of the electrochemical sensor,the signal processing mechanism having data transfer and evaluationsoftware protocols configured to transform raw data from theelectrochemical sensor into diagnostic information regarding thepresence or absence or levels of the target plant volatile compound; and(2) determining the presence of a plant disease associated with thepresence of methyl salicylate based on the information provided by thesignal processing mechanism regarding the presence or absence or levelsof methyl salicylate.